aboutsummaryrefslogtreecommitdiffstats
path: root/Documentation/DocBook/writing-an-alsa-driver.tmpl
blob: 0d0f7b4d4b1a8cad9020274a5644a07c038e6a37 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
2903
2904
2905
2906
2907
2908
2909
2910
2911
2912
2913
2914
2915
2916
2917
2918
2919
2920
2921
2922
2923
2924
2925
2926
2927
2928
2929
2930
2931
2932
2933
2934
2935
2936
2937
2938
2939
2940
2941
2942
2943
2944
2945
2946
2947
2948
2949
2950
2951
2952
2953
2954
2955
2956
2957
2958
2959
2960
2961
2962
2963
2964
2965
2966
2967
2968
2969
2970
2971
2972
2973
2974
2975
2976
2977
2978
2979
2980
2981
2982
2983
2984
2985
2986
2987
2988
2989
2990
2991
2992
2993
2994
2995
2996
2997
2998
2999
3000
3001
3002
3003
3004
3005
3006
3007
3008
3009
3010
3011
3012
3013
3014
3015
3016
3017
3018
3019
3020
3021
3022
3023
3024
3025
3026
3027
3028
3029
3030
3031
3032
3033
3034
3035
3036
3037
3038
3039
3040
3041
3042
3043
3044
3045
3046
3047
3048
3049
3050
3051
3052
3053
3054
3055
3056
3057
3058
3059
3060
3061
3062
3063
3064
3065
3066
3067
3068
3069
3070
3071
3072
3073
3074
3075
3076
3077
3078
3079
3080
3081
3082
3083
3084
3085
3086
3087
3088
3089
3090
3091
3092
3093
3094
3095
3096
3097
3098
3099
3100
3101
3102
3103
3104
3105
3106
3107
3108
3109
3110
3111
3112
3113
3114
3115
3116
3117
3118
3119
3120
3121
3122
3123
3124
3125
3126
3127
3128
3129
3130
3131
3132
3133
3134
3135
3136
3137
3138
3139
3140
3141
3142
3143
3144
3145
3146
3147
3148
3149
3150
3151
3152
3153
3154
3155
3156
3157
3158
3159
3160
3161
3162
3163
3164
3165
3166
3167
3168
3169
3170
3171
3172
3173
3174
3175
3176
3177
3178
3179
3180
3181
3182
3183
3184
3185
3186
3187
3188
3189
3190
3191
3192
3193
3194
3195
3196
3197
3198
3199
3200
3201
3202
3203
3204
3205
3206
3207
3208
3209
3210
3211
3212
3213
3214
3215
3216
3217
3218
3219
3220
3221
3222
3223
3224
3225
3226
3227
3228
3229
3230
3231
3232
3233
3234
3235
3236
3237
3238
3239
3240
3241
3242
3243
3244
3245
3246
3247
3248
3249
3250
3251
3252
3253
3254
3255
3256
3257
3258
3259
3260
3261
3262
3263
3264
3265
3266
3267
3268
3269
3270
3271
3272
3273
3274
3275
3276
3277
3278
3279
3280
3281
3282
3283
3284
3285
3286
3287
3288
3289
3290
3291
3292
3293
3294
3295
3296
3297
3298
3299
3300
3301
3302
3303
3304
3305
3306
3307
3308
3309
3310
3311
3312
3313
3314
3315
3316
3317
3318
3319
3320
3321
3322
3323
3324
3325
3326
3327
3328
3329
3330
3331
3332
3333
3334
3335
3336
3337
3338
3339
3340
3341
3342
3343
3344
3345
3346
3347
3348
3349
3350
3351
3352
3353
3354
3355
3356
3357
3358
3359
3360
3361
3362
3363
3364
3365
3366
3367
3368
3369
3370
3371
3372
3373
3374
3375
3376
3377
3378
3379
3380
3381
3382
3383
3384
3385
3386
3387
3388
3389
3390
3391
3392
3393
3394
3395
3396
3397
3398
3399
3400
3401
3402
3403
3404
3405
3406
3407
3408
3409
3410
3411
3412
3413
3414
3415
3416
3417
3418
3419
3420
3421
3422
3423
3424
3425
3426
3427
3428
3429
3430
3431
3432
3433
3434
3435
3436
3437
3438
3439
3440
3441
3442
3443
3444
3445
3446
3447
3448
3449
3450
3451
3452
3453
3454
3455
3456
3457
3458
3459
3460
3461
3462
3463
3464
3465
3466
3467
3468
3469
3470
3471
3472
3473
3474
3475
3476
3477
3478
3479
3480
3481
3482
3483
3484
3485
3486
3487
3488
3489
3490
3491
3492
3493
3494
3495
3496
3497
3498
3499
3500
3501
3502
3503
3504
3505
3506
3507
3508
3509
3510
3511
3512
3513
3514
3515
3516
3517
3518
3519
3520
3521
3522
3523
3524
3525
3526
3527
3528
3529
3530
3531
3532
3533
3534
3535
3536
3537
3538
3539
3540
3541
3542
3543
3544
3545
3546
3547
3548
3549
3550
3551
3552
3553
3554
3555
3556
3557
3558
3559
3560
3561
3562
3563
3564
3565
3566
3567
3568
3569
3570
3571
3572
3573
3574
3575
3576
3577
3578
3579
3580
3581
3582
3583
3584
3585
3586
3587
3588
3589
3590
3591
3592
3593
3594
3595
3596
3597
3598
3599
3600
3601
3602
3603
3604
3605
3606
3607
3608
3609
3610
3611
3612
3613
3614
3615
3616
3617
3618
3619
3620
3621
3622
3623
3624
3625
3626
3627
3628
3629
3630
3631
3632
3633
3634
3635
3636
3637
3638
3639
3640
3641
3642
3643
3644
3645
3646
3647
3648
3649
3650
3651
3652
3653
3654
3655
3656
3657
3658
3659
3660
3661
3662
3663
3664
3665
3666
3667
3668
3669
3670
3671
3672
3673
3674
3675
3676
3677
3678
3679
3680
3681
3682
3683
3684
3685
3686
3687
3688
3689
3690
3691
3692
3693
3694
3695
3696
3697
3698
3699
3700
3701
3702
3703
3704
3705
3706
3707
3708
3709
3710
3711
3712
3713
3714
3715
3716
3717
3718
3719
3720
3721
3722
3723
3724
3725
3726
3727
3728
3729
3730
3731
3732
3733
3734
3735
3736
3737
3738
3739
3740
3741
3742
3743
3744
3745
3746
3747
3748
3749
3750
3751
3752
3753
3754
3755
3756
3757
3758
3759
3760
3761
3762
3763
3764
3765
3766
3767
3768
3769
3770
3771
3772
3773
3774
3775
3776
3777
3778
3779
3780
3781
3782
3783
3784
3785
3786
3787
3788
3789
3790
3791
3792
3793
3794
3795
3796
3797
3798
3799
3800
3801
3802
3803
3804
3805
3806
3807
3808
3809
3810
3811
3812
3813
3814
3815
3816
3817
3818
3819
3820
3821
3822
3823
3824
3825
3826
3827
3828
3829
3830
3831
3832
3833
3834
3835
3836
3837
3838
3839
3840
3841
3842
3843
3844
3845
3846
3847
3848
3849
3850
3851
3852
3853
3854
3855
3856
3857
3858
3859
3860
3861
3862
3863
3864
3865
3866
3867
3868
3869
3870
3871
3872
3873
3874
3875
3876
3877
3878
3879
3880
3881
3882
3883
3884
3885
3886
3887
3888
3889
3890
3891
3892
3893
3894
3895
3896
3897
3898
3899
3900
3901
3902
3903
3904
3905
3906
3907
3908
3909
3910
3911
3912
3913
3914
3915
3916
3917
3918
3919
3920
3921
3922
3923
3924
3925
3926
3927
3928
3929
3930
3931
3932
3933
3934
3935
3936
3937
3938
3939
3940
3941
3942
3943
3944
3945
3946
3947
3948
3949
3950
3951
3952
3953
3954
3955
3956
3957
3958
3959
3960
3961
3962
3963
3964
3965
3966
3967
3968
3969
3970
3971
3972
3973
3974
3975
3976
3977
3978
3979
3980
3981
3982
3983
3984
3985
3986
3987
3988
3989
3990
3991
3992
3993
3994
3995
3996
3997
3998
3999
4000
4001
4002
4003
4004
4005
4006
4007
4008
4009
4010
4011
4012
4013
4014
4015
4016
4017
4018
4019
4020
4021
4022
4023
4024
4025
4026
4027
4028
4029
4030
4031
4032
4033
4034
4035
4036
4037
4038
4039
4040
4041
4042
4043
4044
4045
4046
4047
4048
4049
4050
4051
4052
4053
4054
4055
4056
4057
4058
4059
4060
4061
4062
4063
4064
4065
4066
4067
4068
4069
4070
4071
4072
4073
4074
4075
4076
4077
4078
4079
4080
4081
4082
4083
4084
4085
4086
4087
4088
4089
4090
4091
4092
4093
4094
4095
4096
4097
4098
4099
4100
4101
4102
4103
4104
4105
4106
4107
4108
4109
4110
4111
4112
4113
4114
4115
4116
4117
4118
4119
4120
4121
4122
4123
4124
4125
4126
4127
4128
4129
4130
4131
4132
4133
4134
4135
4136
4137
4138
4139
4140
4141
4142
4143
4144
4145
4146
4147
4148
4149
4150
4151
4152
4153
4154
4155
4156
4157
4158
4159
4160
4161
4162
4163
4164
4165
4166
4167
4168
4169
4170
4171
4172
4173
4174
4175
4176
4177
4178
4179
4180
4181
4182
4183
4184
4185
4186
4187
4188
4189
4190
4191
4192
4193
4194
4195
4196
4197
4198
4199
4200
4201
4202
4203
4204
4205
4206
4207
4208
4209
4210
4211
4212
4213
4214
4215
4216
4217
4218
4219
4220
4221
4222
4223
4224
4225
4226
4227
4228
4229
4230
4231
4232
4233
4234
4235
4236
4237
4238
4239
4240
4241
4242
4243
4244
4245
4246
4247
4248
4249
4250
4251
4252
4253
4254
4255
4256
4257
4258
4259
4260
4261
4262
4263
4264
4265
4266
4267
4268
4269
4270
4271
4272
4273
4274
4275
4276
4277
4278
4279
4280
4281
4282
4283
4284
4285
4286
4287
4288
4289
4290
4291
4292
4293
4294
4295
4296
4297
4298
4299
4300
4301
4302
4303
4304
4305
4306
4307
4308
4309
4310
4311
4312
4313
4314
4315
4316
4317
4318
4319
4320
4321
4322
4323
4324
4325
4326
4327
4328
4329
4330
4331
4332
4333
4334
4335
4336
4337
4338
4339
4340
4341
4342
4343
4344
4345
4346
4347
4348
4349
4350
4351
4352
4353
4354
4355
4356
4357
4358
4359
4360
4361
4362
4363
4364
4365
4366
4367
4368
4369
4370
4371
4372
4373
4374
4375
4376
4377
4378
4379
4380
4381
4382
4383
4384
4385
4386
4387
4388
4389
4390
4391
4392
4393
4394
4395
4396
4397
4398
4399
4400
4401
4402
4403
4404
4405
4406
4407
4408
4409
4410
4411
4412
4413
4414
4415
4416
4417
4418
4419
4420
4421
4422
4423
4424
4425
4426
4427
4428
4429
4430
4431
4432
4433
4434
4435
4436
4437
4438
4439
4440
4441
4442
4443
4444
4445
4446
4447
4448
4449
4450
4451
4452
4453
4454
4455
4456
4457
4458
4459
4460
4461
4462
4463
4464
4465
4466
4467
4468
4469
4470
4471
4472
4473
4474
4475
4476
4477
4478
4479
4480
4481
4482
4483
4484
4485
4486
4487
4488
4489
4490
4491
4492
4493
4494
4495
4496
4497
4498
4499
4500
4501
4502
4503
4504
4505
4506
4507
4508
4509
4510
4511
4512
4513
4514
4515
4516
4517
4518
4519
4520
4521
4522
4523
4524
4525
4526
4527
4528
4529
4530
4531
4532
4533
4534
4535
4536
4537
4538
4539
4540
4541
4542
4543
4544
4545
4546
4547
4548
4549
4550
4551
4552
4553
4554
4555
4556
4557
4558
4559
4560
4561
4562
4563
4564
4565
4566
4567
4568
4569
4570
4571
4572
4573
4574
4575
4576
4577
4578
4579
4580
4581
4582
4583
4584
4585
4586
4587
4588
4589
4590
4591
4592
4593
4594
4595
4596
4597
4598
4599
4600
4601
4602
4603
4604
4605
4606
4607
4608
4609
4610
4611
4612
4613
4614
4615
4616
4617
4618
4619
4620
4621
4622
4623
4624
4625
4626
4627
4628
4629
4630
4631
4632
4633
4634
4635
4636
4637
4638
4639
4640
4641
4642
4643
4644
4645
4646
4647
4648
4649
4650
4651
4652
4653
4654
4655
4656
4657
4658
4659
4660
4661
4662
4663
4664
4665
4666
4667
4668
4669
4670
4671
4672
4673
4674
4675
4676
4677
4678
4679
4680
4681
4682
4683
4684
4685
4686
4687
4688
4689
4690
4691
4692
4693
4694
4695
4696
4697
4698
4699
4700
4701
4702
4703
4704
4705
4706
4707
4708
4709
4710
4711
4712
4713
4714
4715
4716
4717
4718
4719
4720
4721
4722
4723
4724
4725
4726
4727
4728
4729
4730
4731
4732
4733
4734
4735
4736
4737
4738
4739
4740
4741
4742
4743
4744
4745
4746
4747
4748
4749
4750
4751
4752
4753
4754
4755
4756
4757
4758
4759
4760
4761
4762
4763
4764
4765
4766
4767
4768
4769
4770
4771
4772
4773
4774
4775
4776
4777
4778
4779
4780
4781
4782
4783
4784
4785
4786
4787
4788
4789
4790
4791
4792
4793
4794
4795
4796
4797
4798
4799
4800
4801
4802
4803
4804
4805
4806
4807
4808
4809
4810
4811
4812
4813
4814
4815
4816
4817
4818
4819
4820
4821
4822
4823
4824
4825
4826
4827
4828
4829
4830
4831
4832
4833
4834
4835
4836
4837
4838
4839
4840
4841
4842
4843
4844
4845
4846
4847
4848
4849
4850
4851
4852
4853
4854
4855
4856
4857
4858
4859
4860
4861
4862
4863
4864
4865
4866
4867
4868
4869
4870
4871
4872
4873
4874
4875
4876
4877
4878
4879
4880
4881
4882
4883
4884
4885
4886
4887
4888
4889
4890
4891
4892
4893
4894
4895
4896
4897
4898
4899
4900
4901
4902
4903
4904
4905
4906
4907
4908
4909
4910
4911
4912
4913
4914
4915
4916
4917
4918
4919
4920
4921
4922
4923
4924
4925
4926
4927
4928
4929
4930
4931
4932
4933
4934
4935
4936
4937
4938
4939
4940
4941
4942
4943
4944
4945
4946
4947
4948
4949
4950
4951
4952
4953
4954
4955
4956
4957
4958
4959
4960
4961
4962
4963
4964
4965
4966
4967
4968
4969
4970
4971
4972
4973
4974
4975
4976
4977
4978
4979
4980
4981
4982
4983
4984
4985
4986
4987
4988
4989
4990
4991
4992
4993
4994
4995
4996
4997
4998
4999
5000
5001
5002
5003
5004
5005
5006
5007
5008
5009
5010
5011
5012
5013
5014
5015
5016
5017
5018
5019
5020
5021
5022
5023
5024
5025
5026
5027
5028
5029
5030
5031
5032
5033
5034
5035
5036
5037
5038
5039
5040
5041
5042
5043
5044
5045
5046
5047
5048
5049
5050
5051
5052
5053
5054
5055
5056
5057
5058
5059
5060
5061
5062
5063
5064
5065
5066
5067
5068
5069
5070
5071
5072
5073
5074
5075
5076
5077
5078
5079
5080
5081
5082
5083
5084
5085
5086
5087
5088
5089
5090
5091
5092
5093
5094
5095
5096
5097
5098
5099
5100
5101
5102
5103
5104
5105
5106
5107
5108
5109
5110
5111
5112
5113
5114
5115
5116
5117
5118
5119
5120
5121
5122
5123
5124
5125
5126
5127
5128
5129
5130
5131
5132
5133
5134
5135
5136
5137
5138
5139
5140
5141
5142
5143
5144
5145
5146
5147
5148
5149
5150
5151
5152
5153
5154
5155
5156
5157
5158
5159
5160
5161
5162
5163
5164
5165
5166
5167
5168
5169
5170
5171
5172
5173
5174
5175
5176
5177
5178
5179
5180
5181
5182
5183
5184
5185
5186
5187
5188
5189
5190
5191
5192
5193
5194
5195
5196
5197
5198
5199
5200
5201
5202
5203
5204
5205
5206
5207
5208
5209
5210
5211
5212
5213
5214
5215
5216
5217
5218
5219
5220
5221
5222
5223
5224
5225
5226
5227
5228
5229
5230
5231
5232
5233
5234
5235
5236
5237
5238
5239
5240
5241
5242
5243
5244
5245
5246
5247
5248
5249
5250
5251
5252
5253
5254
5255
5256
5257
5258
5259
5260
5261
5262
5263
5264
5265
5266
5267
5268
5269
5270
5271
5272
5273
5274
5275
5276
5277
5278
5279
5280
5281
5282
5283
5284
5285
5286
5287
5288
5289
5290
5291
5292
5293
5294
5295
5296
5297
5298
5299
5300
5301
5302
5303
5304
5305
5306
5307
5308
5309
5310
5311
5312
5313
5314
5315
5316
5317
5318
5319
5320
5321
5322
5323
5324
5325
5326
5327
5328
5329
5330
5331
5332
5333
5334
5335
5336
5337
5338
5339
5340
5341
5342
5343
5344
5345
5346
5347
5348
5349
5350
5351
5352
5353
5354
5355
5356
5357
5358
5359
5360
5361
5362
5363
5364
5365
5366
5367
5368
5369
5370
5371
5372
5373
5374
5375
5376
5377
5378
5379
5380
5381
5382
5383
5384
5385
5386
5387
5388
5389
5390
5391
5392
5393
5394
5395
5396
5397
5398
5399
5400
5401
5402
5403
5404
5405
5406
5407
5408
5409
5410
5411
5412
5413
5414
5415
5416
5417
5418
5419
5420
5421
5422
5423
5424
5425
5426
5427
5428
5429
5430
5431
5432
5433
5434
5435
5436
5437
5438
5439
5440
5441
5442
5443
5444
5445
5446
5447
5448
5449
5450
5451
5452
5453
5454
5455
5456
5457
5458
5459
5460
5461
5462
5463
5464
5465
5466
5467
5468
5469
5470
5471
5472
5473
5474
5475
5476
5477
5478
5479
5480
5481
5482
5483
5484
5485
5486
5487
5488
5489
5490
5491
5492
5493
5494
5495
5496
5497
5498
5499
5500
5501
5502
5503
5504
5505
5506
5507
5508
5509
5510
5511
5512
5513
5514
5515
5516
5517
5518
5519
5520
5521
5522
5523
5524
5525
5526
5527
5528
5529
5530
5531
5532
5533
5534
5535
5536
5537
5538
5539
5540
5541
5542
5543
5544
5545
5546
5547
5548
5549
5550
5551
5552
5553
5554
5555
5556
5557
5558
5559
5560
5561
5562
5563
5564
5565
5566
5567
5568
5569
5570
5571
5572
5573
5574
5575
5576
5577
5578
5579
5580
5581
5582
5583
5584
5585
5586
5587
5588
5589
5590
5591
5592
5593
5594
5595
5596
5597
5598
5599
5600
5601
5602
5603
5604
5605
5606
5607
5608
5609
5610
5611
5612
5613
5614
5615
5616
5617
5618
5619
5620
5621
5622
5623
5624
5625
5626
5627
5628
5629
5630
5631
5632
5633
5634
5635
5636
5637
5638
5639
5640
5641
5642
5643
5644
5645
5646
5647
5648
5649
5650
5651
5652
5653
5654
5655
5656
5657
5658
5659
5660
5661
5662
5663
5664
5665
5666
5667
5668
5669
5670
5671
5672
5673
5674
5675
5676
5677
5678
5679
5680
5681
5682
5683
5684
5685
5686
5687
5688
5689
5690
5691
5692
5693
5694
5695
5696
5697
5698
5699
5700
5701
5702
5703
5704
5705
5706
5707
5708
5709
5710
5711
5712
5713
5714
5715
5716
5717
5718
5719
5720
5721
5722
5723
5724
5725
5726
5727
5728
5729
5730
5731
5732
5733
5734
5735
5736
5737
5738
5739
5740
5741
5742
5743
5744
5745
5746
5747
5748
5749
5750
5751
5752
5753
5754
5755
5756
5757
5758
5759
5760
5761
5762
5763
5764
5765
5766
5767
5768
5769
5770
5771
5772
5773
5774
5775
5776
5777
5778
5779
5780
5781
5782
5783
5784
5785
5786
5787
5788
5789
5790
5791
5792
5793
5794
5795
5796
5797
5798
5799
5800
5801
5802
5803
5804
5805
5806
5807
5808
5809
5810
5811
5812
5813
5814
5815
5816
5817
5818
5819
5820
5821
5822
5823
5824
5825
5826
5827
5828
5829
5830
5831
5832
5833
5834
5835
5836
5837
5838
5839
5840
5841
5842
5843
5844
5845
5846
5847
5848
5849
5850
5851
5852
5853
5854
5855
5856
5857
5858
5859
5860
5861
5862
5863
5864
5865
5866
5867
5868
5869
5870
5871
5872
5873
5874
5875
5876
5877
5878
5879
5880
5881
5882
5883
5884
5885
5886
5887
5888
5889
5890
5891
5892
5893
5894
5895
5896
5897
5898
5899
5900
5901
5902
5903
5904
5905
5906
5907
5908
5909
5910
5911
5912
5913
5914
5915
5916
5917
5918
5919
5920
5921
5922
5923
5924
5925
5926
5927
5928
5929
5930
5931
5932
5933
5934
5935
5936
5937
5938
5939
5940
5941
5942
5943
5944
5945
5946
5947
5948
5949
5950
5951
5952
5953
5954
5955
5956
5957
5958
5959
5960
5961
5962
5963
5964
5965
5966
5967
5968
5969
5970
5971
5972
5973
5974
5975
5976
5977
5978
5979
5980
5981
5982
5983
5984
5985
5986
5987
5988
5989
5990
5991
5992
5993
5994
5995
5996
5997
5998
5999
6000
6001
6002
6003
6004
6005
6006
6007
6008
6009
6010
6011
6012
6013
6014
6015
6016
6017
6018
6019
6020
6021
6022
6023
6024
6025
6026
6027
6028
6029
6030
6031
6032
6033
6034
6035
6036
6037
6038
6039
6040
6041
6042
6043
6044
6045
6046
6047
6048
6049
6050
6051
6052
6053
6054
6055
6056
6057
6058
6059
6060
6061
6062
6063
6064
6065
6066
6067
6068
6069
6070
6071
6072
6073
6074
6075
6076
6077
6078
6079
6080
6081
6082
6083
6084
6085
6086
6087
6088
6089
6090
6091
6092
6093
6094
6095
6096
6097
6098
6099
6100
6101
6102
6103
6104
6105
6106
6107
6108
6109
6110
6111
6112
6113
6114
6115
6116
6117
6118
6119
6120
6121
6122
6123
6124
6125
6126
6127
6128
6129
6130
6131
6132
6133
6134
6135
6136
6137
6138
6139
6140
6141
6142
6143
6144
6145
6146
6147
6148
6149
6150
6151
6152
6153
6154
6155
6156
6157
6158
6159
6160
6161
6162
6163
6164
6165
6166
6167
6168
6169
6170
6171
6172
6173
6174
6175
6176
6177
6178
6179
6180
6181
6182
6183
6184
6185
6186
6187
6188
6189
6190
6191
6192
6193
6194
6195
6196
6197
6198
6199
6200
6201
6202
6203
6204
6205
6206
6207
6208
6209
6210
6211
6212
6213
6214
6215
6216
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
	"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>

<!-- ****************************************************** -->
<!-- Header  -->
<!-- ****************************************************** -->
<book id="Writing-an-ALSA-Driver">
  <bookinfo>
    <title>Writing an ALSA Driver</title>
    <author>
      <firstname>Takashi</firstname>
      <surname>Iwai</surname>
      <affiliation>
        <address>
          <email>tiwai@suse.de</email>
        </address>
      </affiliation>
     </author>

     <date>Oct 15, 2007</date>
     <edition>0.3.7</edition>

    <abstract>
      <para>
        This document describes how to write an ALSA (Advanced Linux
        Sound Architecture) driver.
      </para>
    </abstract>

    <legalnotice>
    <para>
    Copyright (c) 2002-2005  Takashi Iwai <email>tiwai@suse.de</email>
    </para>

    <para>
    This document is free; you can redistribute it and/or modify it
    under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version. 
    </para>

    <para>
    This document is distributed in the hope that it will be useful,
    but <emphasis>WITHOUT ANY WARRANTY</emphasis>; without even the
    implied warranty of <emphasis>MERCHANTABILITY or FITNESS FOR A
    PARTICULAR PURPOSE</emphasis>. See the GNU General Public License
    for more details.
    </para>

    <para>
    You should have received a copy of the GNU General Public
    License along with this program; if not, write to the Free
    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
    MA 02111-1307 USA
    </para>
    </legalnotice>

  </bookinfo>

<!-- ****************************************************** -->
<!-- Preface  -->
<!-- ****************************************************** -->
  <preface id="preface">
    <title>Preface</title>
    <para>
      This document describes how to write an
      <ulink url="http://www.alsa-project.org/"><citetitle>
      ALSA (Advanced Linux Sound Architecture)</citetitle></ulink>
      driver. The document focuses mainly on PCI soundcards.
      In the case of other device types, the API might
      be different, too. However, at least the ALSA kernel API is
      consistent, and therefore it would be still a bit help for
      writing them.
    </para>

    <para>
    This document targets people who already have enough
    C language skills and have basic linux kernel programming
    knowledge.  This document doesn't explain the general
    topic of linux kernel coding and doesn't cover low-level
    driver implementation details. It only describes
    the standard way to write a PCI sound driver on ALSA.
    </para>

    <para>
      If you are already familiar with the older ALSA ver.0.5.x API, you
    can check the drivers such as <filename>sound/pci/es1938.c</filename> or
    <filename>sound/pci/maestro3.c</filename> which have also almost the same
    code-base in the ALSA 0.5.x tree, so you can compare the differences.
    </para>

    <para>
      This document is still a draft version. Any feedback and
    corrections, please!!
    </para>
  </preface>


<!-- ****************************************************** -->
<!-- File Tree Structure  -->
<!-- ****************************************************** -->
  <chapter id="file-tree">
    <title>File Tree Structure</title>

    <section id="file-tree-general">
      <title>General</title>
      <para>
        The ALSA drivers are provided in two ways.
      </para>

      <para>
        One is the trees provided as a tarball or via cvs from the
      ALSA's ftp site, and another is the 2.6 (or later) Linux kernel
      tree. To synchronize both, the ALSA driver tree is split into
      two different trees: alsa-kernel and alsa-driver. The former
      contains purely the source code for the Linux 2.6 (or later)
      tree. This tree is designed only for compilation on 2.6 or
      later environment. The latter, alsa-driver, contains many subtle
      files for compiling ALSA drivers outside of the Linux kernel tree,
      wrapper functions for older 2.2 and 2.4 kernels, to adapt the latest kernel API,
      and additional drivers which are still in development or in
      tests.  The drivers in alsa-driver tree will be moved to
      alsa-kernel (and eventually to the 2.6 kernel tree) when they are
      finished and confirmed to work fine.
      </para>

      <para>
        The file tree structure of ALSA driver is depicted below. Both
        alsa-kernel and alsa-driver have almost the same file
        structure, except for <quote>core</quote> directory. It's
        named as <quote>acore</quote> in alsa-driver tree. 

        <example>
          <title>ALSA File Tree Structure</title>
          <literallayout>
        sound
                /core
                        /oss
                        /seq
                                /oss
                                /instr
                /ioctl32
                /include
                /drivers
                        /mpu401
                        /opl3
                /i2c
                        /l3
                /synth
                        /emux
                /pci
                        /(cards)
                /isa
                        /(cards)
                /arm
                /ppc
                /sparc
                /usb
                /pcmcia /(cards)
                /oss
          </literallayout>
        </example>
      </para>
    </section>

    <section id="file-tree-core-directory">
      <title>core directory</title>
      <para>
        This directory contains the middle layer which is the heart
      of ALSA drivers. In this directory, the native ALSA modules are
      stored. The sub-directories contain different modules and are
      dependent upon the kernel config. 
      </para>

      <section id="file-tree-core-directory-oss">
        <title>core/oss</title>

        <para>
          The codes for PCM and mixer OSS emulation modules are stored
        in this directory. The rawmidi OSS emulation is included in
        the ALSA rawmidi code since it's quite small. The sequencer
        code is stored in <filename>core/seq/oss</filename> directory (see
        <link linkend="file-tree-core-directory-seq-oss"><citetitle>
        below</citetitle></link>).
        </para>
      </section>

      <section id="file-tree-core-directory-ioctl32">
        <title>core/ioctl32</title>

        <para>
          This directory contains the 32bit-ioctl wrappers for 64bit
        architectures such like x86-64, ppc64 and sparc64. For 32bit
        and alpha architectures, these are not compiled. 
        </para>
      </section>

      <section id="file-tree-core-directory-seq">
        <title>core/seq</title>
        <para>
          This directory and its sub-directories are for the ALSA
        sequencer. This directory contains the sequencer core and
        primary sequencer modules such like snd-seq-midi,
        snd-seq-virmidi, etc. They are compiled only when
        <constant>CONFIG_SND_SEQUENCER</constant> is set in the kernel
        config. 
        </para>
      </section>

      <section id="file-tree-core-directory-seq-oss">
        <title>core/seq/oss</title>
        <para>
          This contains the OSS sequencer emulation codes.
        </para>
      </section>

      <section id="file-tree-core-directory-deq-instr">
        <title>core/seq/instr</title>
        <para>
          This directory contains the modules for the sequencer
        instrument layer. 
        </para>
      </section>
    </section>

    <section id="file-tree-include-directory">
      <title>include directory</title>
      <para>
        This is the place for the public header files of ALSA drivers,
      which are to be exported to user-space, or included by
      several files at different directories. Basically, the private
      header files should not be placed in this directory, but you may
      still find files there, due to historical reasons :) 
      </para>
    </section>

    <section id="file-tree-drivers-directory">
      <title>drivers directory</title>
      <para>
        This directory contains code shared among different drivers
      on different architectures.  They are hence supposed not to be
      architecture-specific.
      For example, the dummy pcm driver and the serial MIDI
      driver are found in this directory. In the sub-directories,
      there is code for components which are independent from
      bus and cpu architectures. 
      </para>

      <section id="file-tree-drivers-directory-mpu401">
        <title>drivers/mpu401</title>
        <para>
          The MPU401 and MPU401-UART modules are stored here.
        </para>
      </section>

      <section id="file-tree-drivers-directory-opl3">
        <title>drivers/opl3 and opl4</title>
        <para>
          The OPL3 and OPL4 FM-synth stuff is found here.
        </para>
      </section>
    </section>

    <section id="file-tree-i2c-directory">
      <title>i2c directory</title>
      <para>
        This contains the ALSA i2c components.
      </para>

      <para>
        Although there is a standard i2c layer on Linux, ALSA has its
      own i2c code for some cards, because the soundcard needs only a
      simple operation and the standard i2c API is too complicated for
      such a purpose. 
      </para>

      <section id="file-tree-i2c-directory-l3">
        <title>i2c/l3</title>
        <para>
          This is a sub-directory for ARM L3 i2c.
        </para>
      </section>
    </section>

    <section id="file-tree-synth-directory">
        <title>synth directory</title>
        <para>
          This contains the synth middle-level modules.
        </para>

        <para>
          So far, there is only Emu8000/Emu10k1 synth driver under
        the <filename>synth/emux</filename> sub-directory. 
        </para>
    </section>

    <section id="file-tree-pci-directory">
      <title>pci directory</title>
      <para>
        This directory and its sub-directories hold the top-level card modules
      for PCI soundcards and the code specific to the PCI BUS.
      </para>

      <para>
        The drivers compiled from a single file are stored directly
      in the pci directory, while the drivers with several source files are
      stored on their own sub-directory (e.g. emu10k1, ice1712). 
      </para>
    </section>

    <section id="file-tree-isa-directory">
      <title>isa directory</title>
      <para>
        This directory and its sub-directories hold the top-level card modules
      for ISA soundcards. 
      </para>
    </section>

    <section id="file-tree-arm-ppc-sparc-directories">
      <title>arm, ppc, and sparc directories</title>
      <para>
        They are used for top-level card modules which are
      specific to one of these architectures. 
      </para>
    </section>

    <section id="file-tree-usb-directory">
      <title>usb directory</title>
      <para>
        This directory contains the USB-audio driver. In the latest version, the
      USB MIDI driver is integrated in the usb-audio driver. 
      </para>
    </section>

    <section id="file-tree-pcmcia-directory">
      <title>pcmcia directory</title>
      <para>
        The PCMCIA, especially PCCard drivers will go here. CardBus
      drivers will be in the pci directory, because their API is identical
      to that of standard PCI cards. 
      </para>
    </section>

    <section id="file-tree-oss-directory">
      <title>oss directory</title>
      <para>
        The OSS/Lite source files are stored here in Linux 2.6 (or
      later) tree. In the ALSA driver tarball, this directory is empty,
      of course :) 
      </para>
    </section>
  </chapter>


<!-- ****************************************************** -->
<!-- Basic Flow for PCI Drivers  -->
<!-- ****************************************************** -->
  <chapter id="basic-flow">
    <title>Basic Flow for PCI Drivers</title>

    <section id="basic-flow-outline">
      <title>Outline</title>
      <para>
        The minimum flow for PCI soundcards is as follows:

        <itemizedlist>
          <listitem><para>define the PCI ID table (see the section
          <link linkend="pci-resource-entries"><citetitle>PCI Entries
          </citetitle></link>).</para></listitem> 
          <listitem><para>create <function>probe()</function> callback.</para></listitem>
          <listitem><para>create <function>remove()</function> callback.</para></listitem>
          <listitem><para>create a <structname>pci_driver</structname> structure
	  containing the three pointers above.</para></listitem>
          <listitem><para>create an <function>init()</function> function just calling
	  the <function>pci_register_driver()</function> to register the pci_driver table
	  defined above.</para></listitem>
          <listitem><para>create an <function>exit()</function> function to call
	  the <function>pci_unregister_driver()</function> function.</para></listitem>
        </itemizedlist>
      </para>
    </section>

    <section id="basic-flow-example">
      <title>Full Code Example</title>
      <para>
        The code example is shown below. Some parts are kept
      unimplemented at this moment but will be filled in the
      next sections. The numbers in the comment lines of the
      <function>snd_mychip_probe()</function> function
      refer to details explained in the following section. 

        <example>
          <title>Basic Flow for PCI Drivers - Example</title>
          <programlisting>
<![CDATA[
  #include <linux/init.h>
  #include <linux/pci.h>
  #include <linux/slab.h>
  #include <sound/core.h>
  #include <sound/initval.h>

  /* module parameters (see "Module Parameters") */
  /* SNDRV_CARDS: maximum number of cards supported by this module */
  static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
  static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
  static int enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;

  /* definition of the chip-specific record */
  struct mychip {
          struct snd_card *card;
          /* the rest of the implementation will be in section
           * "PCI Resource Management"
           */
  };

  /* chip-specific destructor
   * (see "PCI Resource Management")
   */
  static int snd_mychip_free(struct mychip *chip)
  {
          .... /* will be implemented later... */
  }

  /* component-destructor
   * (see "Management of Cards and Components")
   */
  static int snd_mychip_dev_free(struct snd_device *device)
  {
          return snd_mychip_free(device->device_data);
  }

  /* chip-specific constructor
   * (see "Management of Cards and Components")
   */
  static int __devinit snd_mychip_create(struct snd_card *card,
                                         struct pci_dev *pci,
                                         struct mychip **rchip)
  {
          struct mychip *chip;
          int err;
          static struct snd_device_ops ops = {
                 .dev_free = snd_mychip_dev_free,
          };

          *rchip = NULL;

          /* check PCI availability here
           * (see "PCI Resource Management")
           */
          ....

          /* allocate a chip-specific data with zero filled */
          chip = kzalloc(sizeof(*chip), GFP_KERNEL);
          if (chip == NULL)
                  return -ENOMEM;

          chip->card = card;

          /* rest of initialization here; will be implemented
           * later, see "PCI Resource Management"
           */
          ....

          err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
          if (err < 0) {
                  snd_mychip_free(chip);
                  return err;
          }

          snd_card_set_dev(card, &pci->dev);

          *rchip = chip;
          return 0;
  }

  /* constructor -- see "Constructor" sub-section */
  static int __devinit snd_mychip_probe(struct pci_dev *pci,
                               const struct pci_device_id *pci_id)
  {
          static int dev;
          struct snd_card *card;
          struct mychip *chip;
          int err;

          /* (1) */
          if (dev >= SNDRV_CARDS)
                  return -ENODEV;
          if (!enable[dev]) {
                  dev++;
                  return -ENOENT;
          }

          /* (2) */
          err = snd_card_create(index[dev], id[dev], THIS_MODULE, 0, &card);
          if (err < 0)
                  return err;

          /* (3) */
          err = snd_mychip_create(card, pci, &chip);
          if (err < 0) {
                  snd_card_free(card);
                  return err;
          }

          /* (4) */
          strcpy(card->driver, "My Chip");
          strcpy(card->shortname, "My Own Chip 123");
          sprintf(card->longname, "%s at 0x%lx irq %i",
                  card->shortname, chip->ioport, chip->irq);

          /* (5) */
          .... /* implemented later */

          /* (6) */
          err = snd_card_register(card);
          if (err < 0) {
                  snd_card_free(card);
                  return err;
          }

          /* (7) */
          pci_set_drvdata(pci, card);
          dev++;
          return 0;
  }

  /* destructor -- see the "Destructor" sub-section */
  static void __devexit snd_mychip_remove(struct pci_dev *pci)
  {
          snd_card_free(pci_get_drvdata(pci));
          pci_set_drvdata(pci, NULL);
  }
]]>
          </programlisting>
        </example>
      </para>
    </section>

    <section id="basic-flow-constructor">
      <title>Constructor</title>
      <para>
        The real constructor of PCI drivers is the <function>probe</function> callback.
      The <function>probe</function> callback and other component-constructors which are called
      from the <function>probe</function> callback should be defined with
      the <parameter>__devinit</parameter> prefix. You 
      cannot use the <parameter>__init</parameter> prefix for them,
      because any PCI device could be a hotplug device. 
      </para>

      <para>
        In the <function>probe</function> callback, the following scheme is often used.
      </para>

      <section id="basic-flow-constructor-device-index">
        <title>1) Check and increment the device index.</title>
        <para>
          <informalexample>
            <programlisting>
<![CDATA[
  static int dev;
  ....
  if (dev >= SNDRV_CARDS)
          return -ENODEV;
  if (!enable[dev]) {
          dev++;
          return -ENOENT;
  }
]]>
            </programlisting>
          </informalexample>

        where enable[dev] is the module option.
        </para>

        <para>
          Each time the <function>probe</function> callback is called, check the
        availability of the device. If not available, simply increment
        the device index and returns. dev will be incremented also
        later (<link
        linkend="basic-flow-constructor-set-pci"><citetitle>step
        7</citetitle></link>). 
        </para>
      </section>

      <section id="basic-flow-constructor-create-card">
        <title>2) Create a card instance</title>
        <para>
          <informalexample>
            <programlisting>
<![CDATA[
  struct snd_card *card;
  int err;
  ....
  err = snd_card_create(index[dev], id[dev], THIS_MODULE, 0, &card);
]]>
            </programlisting>
          </informalexample>
        </para>

        <para>
          The details will be explained in the section
          <link linkend="card-management-card-instance"><citetitle>
          Management of Cards and Components</citetitle></link>.
        </para>
      </section>

      <section id="basic-flow-constructor-create-main">
        <title>3) Create a main component</title>
        <para>
          In this part, the PCI resources are allocated.

          <informalexample>
            <programlisting>
<![CDATA[
  struct mychip *chip;
  ....
  err = snd_mychip_create(card, pci, &chip);
  if (err < 0) {
          snd_card_free(card);
          return err;
  }
]]>
            </programlisting>
          </informalexample>

          The details will be explained in the section <link
        linkend="pci-resource"><citetitle>PCI Resource
        Management</citetitle></link>.
        </para>
      </section>

      <section id="basic-flow-constructor-main-component">
        <title>4) Set the driver ID and name strings.</title>
        <para>
          <informalexample>
            <programlisting>
<![CDATA[
  strcpy(card->driver, "My Chip");
  strcpy(card->shortname, "My Own Chip 123");
  sprintf(card->longname, "%s at 0x%lx irq %i",
          card->shortname, chip->ioport, chip->irq);
]]>
            </programlisting>
          </informalexample>

          The driver field holds the minimal ID string of the
        chip. This is used by alsa-lib's configurator, so keep it
        simple but unique. 
          Even the same driver can have different driver IDs to
        distinguish the functionality of each chip type. 
        </para>

        <para>
          The shortname field is a string shown as more verbose
        name. The longname field contains the information
        shown in <filename>/proc/asound/cards</filename>. 
        </para>
      </section>

      <section id="basic-flow-constructor-create-other">
        <title>5) Create other components, such as mixer, MIDI, etc.</title>
        <para>
          Here you define the basic components such as
          <link linkend="pcm-interface"><citetitle>PCM</citetitle></link>,
          mixer (e.g. <link linkend="api-ac97"><citetitle>AC97</citetitle></link>),
          MIDI (e.g. <link linkend="midi-interface"><citetitle>MPU-401</citetitle></link>),
          and other interfaces.
          Also, if you want a <link linkend="proc-interface"><citetitle>proc
        file</citetitle></link>, define it here, too.
        </para>
      </section>

      <section id="basic-flow-constructor-register-card">
        <title>6) Register the card instance.</title>
        <para>
          <informalexample>
            <programlisting>
<![CDATA[
  err = snd_card_register(card);
  if (err < 0) {
          snd_card_free(card);
          return err;
  }
]]>
            </programlisting>
          </informalexample>
        </para>

        <para>
          Will be explained in the section <link
        linkend="card-management-registration"><citetitle>Management
        of Cards and Components</citetitle></link>, too. 
        </para>
      </section>

      <section id="basic-flow-constructor-set-pci">
        <title>7) Set the PCI driver data and return zero.</title>
        <para>
          <informalexample>
            <programlisting>
<![CDATA[
        pci_set_drvdata(pci, card);
        dev++;
        return 0;
]]>
            </programlisting>
          </informalexample>

          In the above, the card record is stored. This pointer is
        used in the remove callback and power-management
        callbacks, too. 
        </para>
      </section>
    </section>

    <section id="basic-flow-destructor">
      <title>Destructor</title>
      <para>
        The destructor, remove callback, simply releases the card
      instance. Then the ALSA middle layer will release all the
      attached components automatically. 
      </para>

      <para>
        It would be typically like the following:

        <informalexample>
          <programlisting>
<![CDATA[
  static void __devexit snd_mychip_remove(struct pci_dev *pci)
  {
          snd_card_free(pci_get_drvdata(pci));
          pci_set_drvdata(pci, NULL);
  }
]]>
          </programlisting>
        </informalexample>

        The above code assumes that the card pointer is set to the PCI
	driver data.
      </para>
    </section>

    <section id="basic-flow-header-files">
      <title>Header Files</title>
      <para>
        For the above example, at least the following include files
      are necessary. 

        <informalexample>
          <programlisting>
<![CDATA[
  #include <linux/init.h>
  #include <linux/pci.h>
  #include <linux/slab.h>
  #include <sound/core.h>
  #include <sound/initval.h>
]]>
          </programlisting>
        </informalexample>

	where the last one is necessary only when module options are
      defined in the source file.  If the code is split into several
      files, the files without module options don't need them.
      </para>

      <para>
        In addition to these headers, you'll need
      <filename>&lt;linux/interrupt.h&gt;</filename> for interrupt
      handling, and <filename>&lt;asm/io.h&gt;</filename> for I/O
      access. If you use the <function>mdelay()</function> or
      <function>udelay()</function> functions, you'll need to include
      <filename>&lt;linux/delay.h&gt;</filename> too. 
      </para>

      <para>
      The ALSA interfaces like the PCM and control APIs are defined in other
      <filename>&lt;sound/xxx.h&gt;</filename> header files.
      They have to be included after
      <filename>&lt;sound/core.h&gt;</filename>.
      </para>

    </section>
  </chapter>


<!-- ****************************************************** -->
<!-- Management of Cards and Components  -->
<!-- ****************************************************** -->
  <chapter id="card-management">
    <title>Management of Cards and Components</title>

    <section id="card-management-card-instance">
      <title>Card Instance</title>
      <para>
      For each soundcard, a <quote>card</quote> record must be allocated.
      </para>

      <para>
      A card record is the headquarters of the soundcard.  It manages
      the whole list of devices (components) on the soundcard, such as
      PCM, mixers, MIDI, synthesizer, and so on.  Also, the card
      record holds the ID and the name strings of the card, manages
      the root of proc files, and controls the power-management states
      and hotplug disconnections.  The component list on the card
      record is used to manage the correct release of resources at
      destruction. 
      </para>

      <para>
        As mentioned above, to create a card instance, call
      <function>snd_card_create()</function>.

        <informalexample>
          <programlisting>
<![CDATA[
  struct snd_card *card;
  int err;
  err = snd_card_create(index, id, module, extra_size, &card);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        The function takes five arguments, the card-index number, the
        id string, the module pointer (usually
        <constant>THIS_MODULE</constant>),
        the size of extra-data space, and the pointer to return the
        card instance.  The extra_size argument is used to
        allocate card-&gt;private_data for the
        chip-specific data.  Note that these data
        are allocated by <function>snd_card_create()</function>.
      </para>
    </section>

    <section id="card-management-component">
      <title>Components</title>
      <para>
        After the card is created, you can attach the components
      (devices) to the card instance. In an ALSA driver, a component is
      represented as a struct <structname>snd_device</structname> object.
      A component can be a PCM instance, a control interface, a raw
      MIDI interface, etc.  Each such instance has one component
      entry.
      </para>

      <para>
        A component can be created via
        <function>snd_device_new()</function> function. 

        <informalexample>
          <programlisting>
<![CDATA[
  snd_device_new(card, SNDRV_DEV_XXX, chip, &ops);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        This takes the card pointer, the device-level
      (<constant>SNDRV_DEV_XXX</constant>), the data pointer, and the
      callback pointers (<parameter>&amp;ops</parameter>). The
      device-level defines the type of components and the order of
      registration and de-registration.  For most components, the
      device-level is already defined.  For a user-defined component,
      you can use <constant>SNDRV_DEV_LOWLEVEL</constant>.
      </para>

      <para>
      This function itself doesn't allocate the data space. The data
      must be allocated manually beforehand, and its pointer is passed
      as the argument. This pointer is used as the
      (<parameter>chip</parameter> identifier in the above example)
      for the instance. 
      </para>

      <para>
        Each pre-defined ALSA component such as ac97 and pcm calls
      <function>snd_device_new()</function> inside its
      constructor. The destructor for each component is defined in the
      callback pointers.  Hence, you don't need to take care of
      calling a destructor for such a component.
      </para>

      <para>
        If you wish to create your own component, you need to
      set the destructor function to the dev_free callback in
      the <parameter>ops</parameter>, so that it can be released
      automatically via <function>snd_card_free()</function>.
      The next example will show an implementation of chip-specific
      data.
      </para>
    </section>

    <section id="card-management-chip-specific">
      <title>Chip-Specific Data</title>
      <para>
      Chip-specific information, e.g. the I/O port address, its
      resource pointer, or the irq number, is stored in the
      chip-specific record.

        <informalexample>
          <programlisting>
<![CDATA[
  struct mychip {
          ....
  };
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        In general, there are two ways of allocating the chip record.
      </para>

      <section id="card-management-chip-specific-snd-card-new">
        <title>1. Allocating via <function>snd_card_create()</function>.</title>
        <para>
          As mentioned above, you can pass the extra-data-length
	  to the 4th argument of <function>snd_card_create()</function>, i.e.

          <informalexample>
            <programlisting>
<![CDATA[
  err = snd_card_create(index[dev], id[dev], THIS_MODULE,
                        sizeof(struct mychip), &card);
]]>
            </programlisting>
          </informalexample>

          struct <structname>mychip</structname> is the type of the chip record.
        </para>

        <para>
          In return, the allocated record can be accessed as

          <informalexample>
            <programlisting>
<![CDATA[
  struct mychip *chip = card->private_data;
]]>
            </programlisting>
          </informalexample>

          With this method, you don't have to allocate twice.
          The record is released together with the card instance.
        </para>
      </section>

      <section id="card-management-chip-specific-allocate-extra">
        <title>2. Allocating an extra device.</title>

        <para>
          After allocating a card instance via
          <function>snd_card_create()</function> (with
          <constant>0</constant> on the 4th arg), call
          <function>kzalloc()</function>. 

          <informalexample>
            <programlisting>
<![CDATA[
  struct snd_card *card;
  struct mychip *chip;
  err = snd_card_create(index[dev], id[dev], THIS_MODULE, 0, &card);
  .....
  chip = kzalloc(sizeof(*chip), GFP_KERNEL);
]]>
            </programlisting>
          </informalexample>
        </para>

        <para>
          The chip record should have the field to hold the card
          pointer at least, 

          <informalexample>
            <programlisting>
<![CDATA[
  struct mychip {
          struct snd_card *card;
          ....
  };
]]>
            </programlisting>
          </informalexample>
        </para>

        <para>
          Then, set the card pointer in the returned chip instance.

          <informalexample>
            <programlisting>
<![CDATA[
  chip->card = card;
]]>
            </programlisting>
          </informalexample>
        </para>

        <para>
          Next, initialize the fields, and register this chip
          record as a low-level device with a specified
          <parameter>ops</parameter>, 

          <informalexample>
            <programlisting>
<![CDATA[
  static struct snd_device_ops ops = {
          .dev_free =        snd_mychip_dev_free,
  };
  ....
  snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
]]>
            </programlisting>
          </informalexample>

          <function>snd_mychip_dev_free()</function> is the
        device-destructor function, which will call the real
        destructor. 
        </para>

        <para>
          <informalexample>
            <programlisting>
<![CDATA[
  static int snd_mychip_dev_free(struct snd_device *device)
  {
          return snd_mychip_free(device->device_data);
  }
]]>
            </programlisting>
          </informalexample>

          where <function>snd_mychip_free()</function> is the real destructor.
        </para>
      </section>
    </section>

    <section id="card-management-registration">
      <title>Registration and Release</title>
      <para>
        After all components are assigned, register the card instance
      by calling <function>snd_card_register()</function>. Access
      to the device files is enabled at this point. That is, before
      <function>snd_card_register()</function> is called, the
      components are safely inaccessible from external side. If this
      call fails, exit the probe function after releasing the card via
      <function>snd_card_free()</function>. 
      </para>

      <para>
        For releasing the card instance, you can call simply
      <function>snd_card_free()</function>. As mentioned earlier, all
      components are released automatically by this call. 
      </para>

      <para>
        As further notes, the destructors (both
      <function>snd_mychip_dev_free</function> and
      <function>snd_mychip_free</function>) cannot be defined with
      the <parameter>__devexit</parameter> prefix, because they may be
      called from the constructor, too, at the false path. 
      </para>

      <para>
      For a device which allows hotplugging, you can use
      <function>snd_card_free_when_closed</function>.  This one will
      postpone the destruction until all devices are closed.
      </para>

    </section>

  </chapter>


<!-- ****************************************************** -->
<!-- PCI Resource Management  -->
<!-- ****************************************************** -->
  <chapter id="pci-resource">
    <title>PCI Resource Management</title>

    <section id="pci-resource-example">
      <title>Full Code Example</title>
      <para>
        In this section, we'll complete the chip-specific constructor,
      destructor and PCI entries. Example code is shown first,
      below. 

        <example>
          <title>PCI Resource Management Example</title>
          <programlisting>
<![CDATA[
  struct mychip {
          struct snd_card *card;
          struct pci_dev *pci;

          unsigned long port;
          int irq;
  };

  static int snd_mychip_free(struct mychip *chip)
  {
          /* disable hardware here if any */
          .... /* (not implemented in this document) */

          /* release the irq */
          if (chip->irq >= 0)
                  free_irq(chip->irq, chip);
          /* release the I/O ports & memory */
          pci_release_regions(chip->pci);
          /* disable the PCI entry */
          pci_disable_device(chip->pci);
          /* release the data */
          kfree(chip);
          return 0;
  }

  /* chip-specific constructor */
  static int __devinit snd_mychip_create(struct snd_card *card,
                                         struct pci_dev *pci,
                                         struct mychip **rchip)
  {
          struct mychip *chip;
          int err;
          static struct snd_device_ops ops = {
                 .dev_free = snd_mychip_dev_free,
          };

          *rchip = NULL;

          /* initialize the PCI entry */
          err = pci_enable_device(pci);
          if (err < 0)
                  return err;
          /* check PCI availability (28bit DMA) */
          if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
              pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
                  printk(KERN_ERR "error to set 28bit mask DMA\n");
                  pci_disable_device(pci);
                  return -ENXIO;
          }

          chip = kzalloc(sizeof(*chip), GFP_KERNEL);
          if (chip == NULL) {
                  pci_disable_device(pci);
                  return -ENOMEM;
          }

          /* initialize the stuff */
          chip->card = card;
          chip->pci = pci;
          chip->irq = -1;

          /* (1) PCI resource allocation */
          err = pci_request_regions(pci, "My Chip");
          if (err < 0) {
                  kfree(chip);
                  pci_disable_device(pci);
                  return err;
          }
          chip->port = pci_resource_start(pci, 0);
          if (request_irq(pci->irq, snd_mychip_interrupt,
                          IRQF_SHARED, "My Chip", chip)) {
                  printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
                  snd_mychip_free(chip);
                  return -EBUSY;
          }
          chip->irq = pci->irq;

          /* (2) initialization of the chip hardware */
          .... /*   (not implemented in this document) */

          err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
          if (err < 0) {
                  snd_mychip_free(chip);
                  return err;
          }

          snd_card_set_dev(card, &pci->dev);

          *rchip = chip;
          return 0;
  }        

  /* PCI IDs */
  static struct pci_device_id snd_mychip_ids[] = {
          { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
            PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
          ....
          { 0, }
  };
  MODULE_DEVICE_TABLE(pci, snd_mychip_ids);

  /* pci_driver definition */
  static struct pci_driver driver = {
          .name = "My Own Chip",
          .id_table = snd_mychip_ids,
          .probe = snd_mychip_probe,
          .remove = __devexit_p(snd_mychip_remove),
  };

  /* module initialization */
  static int __init alsa_card_mychip_init(void)
  {
          return pci_register_driver(&driver);
  }

  /* module clean up */
  static void __exit alsa_card_mychip_exit(void)
  {
          pci_unregister_driver(&driver);
  }

  module_init(alsa_card_mychip_init)
  module_exit(alsa_card_mychip_exit)

  EXPORT_NO_SYMBOLS; /* for old kernels only */
]]>
          </programlisting>
        </example>
      </para>
    </section>

    <section id="pci-resource-some-haftas">
      <title>Some Hafta's</title>
      <para>
        The allocation of PCI resources is done in the
      <function>probe()</function> function, and usually an extra
      <function>xxx_create()</function> function is written for this
      purpose.
      </para>

      <para>
        In the case of PCI devices, you first have to call
      the <function>pci_enable_device()</function> function before
      allocating resources. Also, you need to set the proper PCI DMA
      mask to limit the accessed I/O range. In some cases, you might
      need to call <function>pci_set_master()</function> function,
      too.
      </para>

      <para>
        Suppose the 28bit mask, and the code to be added would be like:

        <informalexample>
          <programlisting>
<![CDATA[
  err = pci_enable_device(pci);
  if (err < 0)
          return err;
  if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
      pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
          printk(KERN_ERR "error to set 28bit mask DMA\n");
          pci_disable_device(pci);
          return -ENXIO;
  }
  
]]>
          </programlisting>
        </informalexample>
      </para>
    </section>

    <section id="pci-resource-resource-allocation">
      <title>Resource Allocation</title>
      <para>
        The allocation of I/O ports and irqs is done via standard kernel
      functions. Unlike ALSA ver.0.5.x., there are no helpers for
      that. And these resources must be released in the destructor
      function (see below). Also, on ALSA 0.9.x, you don't need to
      allocate (pseudo-)DMA for PCI like in ALSA 0.5.x.
      </para>

      <para>
        Now assume that the PCI device has an I/O port with 8 bytes
        and an interrupt. Then struct <structname>mychip</structname> will have the
        following fields:

        <informalexample>
          <programlisting>
<![CDATA[
  struct mychip {
          struct snd_card *card;

          unsigned long port;
          int irq;
  };
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        For an I/O port (and also a memory region), you need to have
      the resource pointer for the standard resource management. For
      an irq, you have to keep only the irq number (integer). But you
      need to initialize this number as -1 before actual allocation,
      since irq 0 is valid. The port address and its resource pointer
      can be initialized as null by
      <function>kzalloc()</function> automatically, so you
      don't have to take care of resetting them. 
      </para>

      <para>
        The allocation of an I/O port is done like this:

        <informalexample>
          <programlisting>
<![CDATA[
  err = pci_request_regions(pci, "My Chip");
  if (err < 0) { 
          kfree(chip);
          pci_disable_device(pci);
          return err;
  }
  chip->port = pci_resource_start(pci, 0);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        <!-- obsolete -->
        It will reserve the I/O port region of 8 bytes of the given
      PCI device. The returned value, chip-&gt;res_port, is allocated
      via <function>kmalloc()</function> by
      <function>request_region()</function>. The pointer must be
      released via <function>kfree()</function>, but there is a
      problem with this. This issue will be explained later.
      </para>

      <para>
        The allocation of an interrupt source is done like this:

        <informalexample>
          <programlisting>
<![CDATA[
  if (request_irq(pci->irq, snd_mychip_interrupt,
                  IRQF_SHARED, "My Chip", chip)) {
          printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
          snd_mychip_free(chip);
          return -EBUSY;
  }
  chip->irq = pci->irq;
]]>
          </programlisting>
        </informalexample>

        where <function>snd_mychip_interrupt()</function> is the
      interrupt handler defined <link
      linkend="pcm-interface-interrupt-handler"><citetitle>later</citetitle></link>.
      Note that chip-&gt;irq should be defined
      only when <function>request_irq()</function> succeeded.
      </para>

      <para>
      On the PCI bus, interrupts can be shared. Thus,
      <constant>IRQF_SHARED</constant> is used as the interrupt flag of
      <function>request_irq()</function>. 
      </para>

      <para>
        The last argument of <function>request_irq()</function> is the
      data pointer passed to the interrupt handler. Usually, the
      chip-specific record is used for that, but you can use what you
      like, too. 
      </para>

      <para>
        I won't give details about the interrupt handler at this
        point, but at least its appearance can be explained now. The
        interrupt handler looks usually like the following: 

        <informalexample>
          <programlisting>
<![CDATA[
  static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
  {
          struct mychip *chip = dev_id;
          ....
          return IRQ_HANDLED;
  }
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        Now let's write the corresponding destructor for the resources
      above. The role of destructor is simple: disable the hardware
      (if already activated) and release the resources. So far, we
      have no hardware part, so the disabling code is not written here. 
      </para>

      <para>
        To release the resources, the <quote>check-and-release</quote>
        method is a safer way. For the interrupt, do like this: 

        <informalexample>
          <programlisting>
<![CDATA[
  if (chip->irq >= 0)
          free_irq(chip->irq, chip);
]]>
          </programlisting>
        </informalexample>

        Since the irq number can start from 0, you should initialize
        chip-&gt;irq with a negative value (e.g. -1), so that you can
        check the validity of the irq number as above.
      </para>

      <para>
        When you requested I/O ports or memory regions via
	<function>pci_request_region()</function> or
	<function>pci_request_regions()</function> like in this example,
	release the resource(s) using the corresponding function,
	<function>pci_release_region()</function> or
	<function>pci_release_regions()</function>.

        <informalexample>
          <programlisting>
<![CDATA[
  pci_release_regions(chip->pci);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
	When you requested manually via <function>request_region()</function>
	or <function>request_mem_region</function>, you can release it via
	<function>release_resource()</function>.  Suppose that you keep
	the resource pointer returned from <function>request_region()</function>
	in chip-&gt;res_port, the release procedure looks like:

        <informalexample>
          <programlisting>
<![CDATA[
  release_and_free_resource(chip->res_port);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
      Don't forget to call <function>pci_disable_device()</function>
      before the end.
      </para>

      <para>
        And finally, release the chip-specific record.

        <informalexample>
          <programlisting>
<![CDATA[
  kfree(chip);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
      Again, remember that you cannot
      use the <parameter>__devexit</parameter> prefix for this destructor. 
      </para>

      <para>
      We didn't implement the hardware disabling part in the above.
      If you need to do this, please note that the destructor may be
      called even before the initialization of the chip is completed.
      It would be better to have a flag to skip hardware disabling
      if the hardware was not initialized yet.
      </para>

      <para>
      When the chip-data is assigned to the card using
      <function>snd_device_new()</function> with
      <constant>SNDRV_DEV_LOWLELVEL</constant> , its destructor is 
      called at the last.  That is, it is assured that all other
      components like PCMs and controls have already been released.
      You don't have to stop PCMs, etc. explicitly, but just
      call low-level hardware stopping.
      </para>

      <para>
        The management of a memory-mapped region is almost as same as
        the management of an I/O port. You'll need three fields like
        the following: 

        <informalexample>
          <programlisting>
<![CDATA[
  struct mychip {
          ....
          unsigned long iobase_phys;
          void __iomem *iobase_virt;
  };
]]>
          </programlisting>
        </informalexample>

        and the allocation would be like below:

        <informalexample>
          <programlisting>
<![CDATA[
  if ((err = pci_request_regions(pci, "My Chip")) < 0) {
          kfree(chip);
          return err;
  }
  chip->iobase_phys = pci_resource_start(pci, 0);
  chip->iobase_virt = ioremap_nocache(chip->iobase_phys,
                                      pci_resource_len(pci, 0));
]]>
          </programlisting>
        </informalexample>
        
        and the corresponding destructor would be:

        <informalexample>
          <programlisting>
<![CDATA[
  static int snd_mychip_free(struct mychip *chip)
  {
          ....
          if (chip->iobase_virt)
                  iounmap(chip->iobase_virt);
          ....
          pci_release_regions(chip->pci);
          ....
  }
]]>
          </programlisting>
        </informalexample>
      </para>

    </section>

    <section id="pci-resource-device-struct">
      <title>Registration of Device Struct</title>
      <para>
	At some point, typically after calling <function>snd_device_new()</function>,
	you need to register the struct <structname>device</structname> of the chip
	you're handling for udev and co.  ALSA provides a macro for compatibility with
	older kernels.  Simply call like the following:
        <informalexample>
          <programlisting>
<![CDATA[
  snd_card_set_dev(card, &pci->dev);
]]>
          </programlisting>
        </informalexample>
	so that it stores the PCI's device pointer to the card.  This will be
	referred by ALSA core functions later when the devices are registered.
      </para>
      <para>
	In the case of non-PCI, pass the proper device struct pointer of the BUS
	instead.  (In the case of legacy ISA without PnP, you don't have to do
	anything.)
      </para>
    </section>

    <section id="pci-resource-entries">
      <title>PCI Entries</title>
      <para>
        So far, so good. Let's finish the missing PCI
      stuff. At first, we need a
      <structname>pci_device_id</structname> table for this
      chipset. It's a table of PCI vendor/device ID number, and some
      masks. 
      </para>

      <para>
        For example,

        <informalexample>
          <programlisting>
<![CDATA[
  static struct pci_device_id snd_mychip_ids[] = {
          { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
            PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
          ....
          { 0, }
  };
  MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        The first and second fields of
      the <structname>pci_device_id</structname> structure are the vendor and
      device IDs. If you have no reason to filter the matching
      devices, you can leave the remaining fields as above. The last
      field of the <structname>pci_device_id</structname> struct contains
      private data for this entry. You can specify any value here, for
      example, to define specific operations for supported device IDs.
      Such an example is found in the intel8x0 driver. 
      </para>

      <para>
        The last entry of this list is the terminator. You must
      specify this all-zero entry. 
      </para>

      <para>
        Then, prepare the <structname>pci_driver</structname> record:

        <informalexample>
          <programlisting>
<![CDATA[
  static struct pci_driver driver = {
          .name = "My Own Chip",
          .id_table = snd_mychip_ids,
          .probe = snd_mychip_probe,
          .remove = __devexit_p(snd_mychip_remove),
  };
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        The <structfield>probe</structfield> and
      <structfield>remove</structfield> functions have already
      been defined in the previous sections.
      The <structfield>remove</structfield> function should
      be defined with the 
      <function>__devexit_p()</function> macro, so that it's not
      defined for built-in (and non-hot-pluggable) case. The
      <structfield>name</structfield> 
      field is the name string of this device. Note that you must not
      use a slash <quote>/</quote> in this string. 
      </para>

      <para>
        And at last, the module entries:

        <informalexample>
          <programlisting>
<![CDATA[
  static int __init alsa_card_mychip_init(void)
  {
          return pci_register_driver(&driver);
  }

  static void __exit alsa_card_mychip_exit(void)
  {
          pci_unregister_driver(&driver);
  }

  module_init(alsa_card_mychip_init)
  module_exit(alsa_card_mychip_exit)
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        Note that these module entries are tagged with
      <parameter>__init</parameter> and 
      <parameter>__exit</parameter> prefixes, not
      <parameter>__devinit</parameter> nor
      <parameter>__devexit</parameter>.
      </para>

      <para>
        Oh, one thing was forgotten. If you have no exported symbols,
        you need to declare it in 2.2 or 2.4 kernels (it's not necessary in 2.6 kernels).

        <informalexample>
          <programlisting>
<![CDATA[
  EXPORT_NO_SYMBOLS;
]]>
          </programlisting>
        </informalexample>

        That's all!
      </para>
    </section>
  </chapter>


<!-- ****************************************************** -->
<!-- PCM Interface  -->
<!-- ****************************************************** -->
  <chapter id="pcm-interface">
    <title>PCM Interface</title>

    <section id="pcm-interface-general">
      <title>General</title>
      <para>
        The PCM middle layer of ALSA is quite powerful and it is only
      necessary for each driver to implement the low-level functions
      to access its hardware.
      </para>

      <para>
        For accessing to the PCM layer, you need to include
      <filename>&lt;sound/pcm.h&gt;</filename> first. In addition,
      <filename>&lt;sound/pcm_params.h&gt;</filename> might be needed
      if you access to some functions related with hw_param. 
      </para>

      <para>
        Each card device can have up to four pcm instances. A pcm
      instance corresponds to a pcm device file. The limitation of
      number of instances comes only from the available bit size of
      the Linux's device numbers. Once when 64bit device number is
      used, we'll have more pcm instances available. 
      </para>

      <para>
        A pcm instance consists of pcm playback and capture streams,
      and each pcm stream consists of one or more pcm substreams. Some
      soundcards support multiple playback functions. For example,
      emu10k1 has a PCM playback of 32 stereo substreams. In this case, at
      each open, a free substream is (usually) automatically chosen
      and opened. Meanwhile, when only one substream exists and it was
      already opened, the successful open will either block
      or error with <constant>EAGAIN</constant> according to the
      file open mode. But you don't have to care about such details in your
      driver. The PCM middle layer will take care of such work.
      </para>
    </section>

    <section id="pcm-interface-example">
      <title>Full Code Example</title>
      <para>
      The example code below does not include any hardware access
      routines but shows only the skeleton, how to build up the PCM
      interfaces.

        <example>
          <title>PCM Example Code</title>
          <programlisting>
<![CDATA[
  #include <sound/pcm.h>
  ....

  /* hardware definition */
  static struct snd_pcm_hardware snd_mychip_playback_hw = {
          .info = (SNDRV_PCM_INFO_MMAP |
                   SNDRV_PCM_INFO_INTERLEAVED |
                   SNDRV_PCM_INFO_BLOCK_TRANSFER |
                   SNDRV_PCM_INFO_MMAP_VALID),
          .formats =          SNDRV_PCM_FMTBIT_S16_LE,
          .rates =            SNDRV_PCM_RATE_8000_48000,
          .rate_min =         8000,
          .rate_max =         48000,
          .channels_min =     2,
          .channels_max =     2,
          .buffer_bytes_max = 32768,
          .period_bytes_min = 4096,
          .period_bytes_max = 32768,
          .periods_min =      1,
          .periods_max =      1024,
  };

  /* hardware definition */
  static struct snd_pcm_hardware snd_mychip_capture_hw = {
          .info = (SNDRV_PCM_INFO_MMAP |
                   SNDRV_PCM_INFO_INTERLEAVED |
                   SNDRV_PCM_INFO_BLOCK_TRANSFER |
                   SNDRV_PCM_INFO_MMAP_VALID),
          .formats =          SNDRV_PCM_FMTBIT_S16_LE,
          .rates =            SNDRV_PCM_RATE_8000_48000,
          .rate_min =         8000,
          .rate_max =         48000,
          .channels_min =     2,
          .channels_max =     2,
          .buffer_bytes_max = 32768,
          .period_bytes_min = 4096,
          .period_bytes_max = 32768,
          .periods_min =      1,
          .periods_max =      1024,
  };

  /* open callback */
  static int snd_mychip_playback_open(struct snd_pcm_substream *substream)
  {
          struct mychip *chip = snd_pcm_substream_chip(substream);
          struct snd_pcm_runtime *runtime = substream->runtime;

          runtime->hw = snd_mychip_playback_hw;
          /* more hardware-initialization will be done here */
          ....
          return 0;
  }

  /* close callback */
  static int snd_mychip_playback_close(struct snd_pcm_substream *substream)
  {
          struct mychip *chip = snd_pcm_substream_chip(substream);
          /* the hardware-specific codes will be here */
          ....
          return 0;

  }

  /* open callback */
  static int snd_mychip_capture_open(struct snd_pcm_substream *substream)
  {
          struct mychip *chip = snd_pcm_substream_chip(substream);
          struct snd_pcm_runtime *runtime = substream->runtime;

          runtime->hw = snd_mychip_capture_hw;
          /* more hardware-initialization will be done here */
          ....
          return 0;
  }

  /* close callback */
  static int snd_mychip_capture_close(struct snd_pcm_substream *substream)
  {
          struct mychip *chip = snd_pcm_substream_chip(substream);
          /* the hardware-specific codes will be here */
          ....
          return 0;

  }

  /* hw_params callback */
  static int snd_mychip_pcm_hw_params(struct snd_pcm_substream *substream,
                               struct snd_pcm_hw_params *hw_params)
  {
          return snd_pcm_lib_malloc_pages(substream,
                                     params_buffer_bytes(hw_params));
  }

  /* hw_free callback */
  static int snd_mychip_pcm_hw_free(struct snd_pcm_substream *substream)
  {
          return snd_pcm_lib_free_pages(substream);
  }

  /* prepare callback */
  static int snd_mychip_pcm_prepare(struct snd_pcm_substream *substream)
  {
          struct mychip *chip = snd_pcm_substream_chip(substream);
          struct snd_pcm_runtime *runtime = substream->runtime;

          /* set up the hardware with the current configuration
           * for example...
           */
          mychip_set_sample_format(chip, runtime->format);
          mychip_set_sample_rate(chip, runtime->rate);
          mychip_set_channels(chip, runtime->channels);
          mychip_set_dma_setup(chip, runtime->dma_addr,
                               chip->buffer_size,
                               chip->period_size);
          return 0;
  }

  /* trigger callback */
  static int snd_mychip_pcm_trigger(struct snd_pcm_substream *substream,
                                    int cmd)
  {
          switch (cmd) {
          case SNDRV_PCM_TRIGGER_START:
                  /* do something to start the PCM engine */
                  ....
                  break;
          case SNDRV_PCM_TRIGGER_STOP:
                  /* do something to stop the PCM engine */
                  ....
                  break;
          default:
                  return -EINVAL;
          }
  }

  /* pointer callback */
  static snd_pcm_uframes_t
  snd_mychip_pcm_pointer(struct snd_pcm_substream *substream)
  {
          struct mychip *chip = snd_pcm_substream_chip(substream);
          unsigned int current_ptr;

          /* get the current hardware pointer */
          current_ptr = mychip_get_hw_pointer(chip);
          return current_ptr;
  }

  /* operators */
  static struct snd_pcm_ops snd_mychip_playback_ops = {
          .open =        snd_mychip_playback_open,
          .close =       snd_mychip_playback_close,
          .ioctl =       snd_pcm_lib_ioctl,
          .hw_params =   snd_mychip_pcm_hw_params,
          .hw_free =     snd_mychip_pcm_hw_free,
          .prepare =     snd_mychip_pcm_prepare,
          .trigger =     snd_mychip_pcm_trigger,
          .pointer =     snd_mychip_pcm_pointer,
  };

  /* operators */
  static struct snd_pcm_ops snd_mychip_capture_ops = {
          .open =        snd_mychip_capture_open,
          .close =       snd_mychip_capture_close,
          .ioctl =       snd_pcm_lib_ioctl,
          .hw_params =   snd_mychip_pcm_hw_params,
          .hw_free =     snd_mychip_pcm_hw_free,
          .prepare =     snd_mychip_pcm_prepare,
          .trigger =     snd_mychip_pcm_trigger,
          .pointer =     snd_mychip_pcm_pointer,
  };

  /*
   *  definitions of capture are omitted here...
   */

  /* create a pcm device */
  static int __devinit snd_mychip_new_pcm(struct mychip *chip)
  {
          struct snd_pcm *pcm;
          int err;

          err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
          if (err < 0) 
                  return err;
          pcm->private_data = chip;
          strcpy(pcm->name, "My Chip");
          chip->pcm = pcm;
          /* set operators */
          snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
                          &snd_mychip_playback_ops);
          snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
                          &snd_mychip_capture_ops);
          /* pre-allocation of buffers */
          /* NOTE: this may fail */
          snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
                                                snd_dma_pci_data(chip->pci),
                                                64*1024, 64*1024);
          return 0;
  }
]]>
          </programlisting>
        </example>
      </para>
    </section>

    <section id="pcm-interface-constructor">
      <title>Constructor</title>
      <para>
        A pcm instance is allocated by the <function>snd_pcm_new()</function>
      function. It would be better to create a constructor for pcm,
      namely, 

        <informalexample>
          <programlisting>
<![CDATA[
  static int __devinit snd_mychip_new_pcm(struct mychip *chip)
  {
          struct snd_pcm *pcm;
          int err;

          err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
          if (err < 0) 
                  return err;
          pcm->private_data = chip;
          strcpy(pcm->name, "My Chip");
          chip->pcm = pcm;
	  ....
          return 0;
  }
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        The <function>snd_pcm_new()</function> function takes four
      arguments. The first argument is the card pointer to which this
      pcm is assigned, and the second is the ID string. 
      </para>

      <para>
        The third argument (<parameter>index</parameter>, 0 in the
      above) is the index of this new pcm. It begins from zero. If
      you create more than one pcm instances, specify the
      different numbers in this argument. For example,
      <parameter>index</parameter> = 1 for the second PCM device.  
      </para>

      <para>
        The fourth and fifth arguments are the number of substreams
      for playback and capture, respectively. Here 1 is used for
      both arguments. When no playback or capture substreams are available,
      pass 0 to the corresponding argument.
      </para>

      <para>
        If a chip supports multiple playbacks or captures, you can
      specify more numbers, but they must be handled properly in
      open/close, etc. callbacks.  When you need to know which
      substream you are referring to, then it can be obtained from
      struct <structname>snd_pcm_substream</structname> data passed to each callback
      as follows: 

        <informalexample>
          <programlisting>
<![CDATA[
  struct snd_pcm_substream *substream;
  int index = substream->number;
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        After the pcm is created, you need to set operators for each
        pcm stream. 

        <informalexample>
          <programlisting>
<![CDATA[
  snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
                  &snd_mychip_playback_ops);
  snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
                  &snd_mychip_capture_ops);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        The operators are defined typically like this:

        <informalexample>
          <programlisting>
<![CDATA[
  static struct snd_pcm_ops snd_mychip_playback_ops = {
          .open =        snd_mychip_pcm_open,
          .close =       snd_mychip_pcm_close,
          .ioctl =       snd_pcm_lib_ioctl,
          .hw_params =   snd_mychip_pcm_hw_params,
          .hw_free =     snd_mychip_pcm_hw_free,
          .prepare =     snd_mychip_pcm_prepare,
          .trigger =     snd_mychip_pcm_trigger,
          .pointer =     snd_mychip_pcm_pointer,
  };
]]>
          </programlisting>
        </informalexample>

        All the callbacks are described in the
        <link linkend="pcm-interface-operators"><citetitle>
        Operators</citetitle></link> subsection.
      </para>

      <para>
        After setting the operators, you probably will want to
        pre-allocate the buffer. For the pre-allocation, simply call
        the following: 

        <informalexample>
          <programlisting>
<![CDATA[
  snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
                                        snd_dma_pci_data(chip->pci),
                                        64*1024, 64*1024);
]]>
          </programlisting>
        </informalexample>

        It will allocate a buffer up to 64kB as default.
      Buffer management details will be described in the later section <link
      linkend="buffer-and-memory"><citetitle>Buffer and Memory
      Management</citetitle></link>. 
      </para>

      <para>
        Additionally, you can set some extra information for this pcm
        in pcm-&gt;info_flags.
        The available values are defined as
        <constant>SNDRV_PCM_INFO_XXX</constant> in
        <filename>&lt;sound/asound.h&gt;</filename>, which is used for
        the hardware definition (described later). When your soundchip
        supports only half-duplex, specify like this: 

        <informalexample>
          <programlisting>
<![CDATA[
  pcm->info_flags = SNDRV_PCM_INFO_HALF_DUPLEX;
]]>
          </programlisting>
        </informalexample>
      </para>
    </section>

    <section id="pcm-interface-destructor">
      <title>... And the Destructor?</title>
      <para>
        The destructor for a pcm instance is not always
      necessary. Since the pcm device will be released by the middle
      layer code automatically, you don't have to call the destructor
      explicitly.
      </para>

      <para>
        The destructor would be necessary if you created
        special records internally and needed to release them. In such a
        case, set the destructor function to
        pcm-&gt;private_free: 

        <example>
          <title>PCM Instance with a Destructor</title>
          <programlisting>
<![CDATA[
  static void mychip_pcm_free(struct snd_pcm *pcm)
  {
          struct mychip *chip = snd_pcm_chip(pcm);
          /* free your own data */
          kfree(chip->my_private_pcm_data);
          /* do what you like else */
          ....
  }

  static int __devinit snd_mychip_new_pcm(struct mychip *chip)
  {
          struct snd_pcm *pcm;
          ....
          /* allocate your own data */
          chip->my_private_pcm_data = kmalloc(...);
          /* set the destructor */
          pcm->private_data = chip;
          pcm->private_free = mychip_pcm_free;
          ....
  }
]]>
          </programlisting>
        </example>
      </para>
    </section>

    <section id="pcm-interface-runtime">
      <title>Runtime Pointer - The Chest of PCM Information</title>
	<para>
	  When the PCM substream is opened, a PCM runtime instance is
	allocated and assigned to the substream. This pointer is
	accessible via <constant>substream-&gt;runtime</constant>.
	This runtime pointer holds most information you need
	to control the PCM: the copy of hw_params and sw_params configurations, the buffer
	pointers, mmap records, spinlocks, etc.
	</para>

	<para>
	The definition of runtime instance is found in
	<filename>&lt;sound/pcm.h&gt;</filename>.  Here are
       the contents of this file:
          <informalexample>
            <programlisting>
<![CDATA[
struct _snd_pcm_runtime {
	/* -- Status -- */
	struct snd_pcm_substream *trigger_master;
	snd_timestamp_t trigger_tstamp;	/* trigger timestamp */
	int overrange;
	snd_pcm_uframes_t avail_max;
	snd_pcm_uframes_t hw_ptr_base;	/* Position at buffer restart */
	snd_pcm_uframes_t hw_ptr_interrupt; /* Position at interrupt time*/

	/* -- HW params -- */
	snd_pcm_access_t access;	/* access mode */
	snd_pcm_format_t format;	/* SNDRV_PCM_FORMAT_* */
	snd_pcm_subformat_t subformat;	/* subformat */
	unsigned int rate;		/* rate in Hz */
	unsigned int channels;		/* channels */
	snd_pcm_uframes_t period_size;	/* period size */
	unsigned int periods;		/* periods */
	snd_pcm_uframes_t buffer_size;	/* buffer size */
	unsigned int tick_time;		/* tick time */
	snd_pcm_uframes_t min_align;	/* Min alignment for the format */
	size_t byte_align;
	unsigned int frame_bits;
	unsigned int sample_bits;
	unsigned int info;
	unsigned int rate_num;
	unsigned int rate_den;

	/* -- SW params -- */
	struct timespec tstamp_mode;	/* mmap timestamp is updated */
  	unsigned int period_step;
	unsigned int sleep_min;		/* min ticks to sleep */
	snd_pcm_uframes_t start_threshold;
	snd_pcm_uframes_t stop_threshold;
	snd_pcm_uframes_t silence_threshold; /* Silence filling happens when
						noise is nearest than this */
	snd_pcm_uframes_t silence_size;	/* Silence filling size */
	snd_pcm_uframes_t boundary;	/* pointers wrap point */

	snd_pcm_uframes_t silenced_start;
	snd_pcm_uframes_t silenced_size;

	snd_pcm_sync_id_t sync;		/* hardware synchronization ID */

	/* -- mmap -- */
	volatile struct snd_pcm_mmap_status *status;
	volatile struct snd_pcm_mmap_control *control;
	atomic_t mmap_count;

	/* -- locking / scheduling -- */
	spinlock_t lock;
	wait_queue_head_t sleep;
	struct timer_list tick_timer;
	struct fasync_struct *fasync;

	/* -- private section -- */
	void *private_data;
	void (*private_free)(struct snd_pcm_runtime *runtime);

	/* -- hardware description -- */
	struct snd_pcm_hardware hw;
	struct snd_pcm_hw_constraints hw_constraints;

	/* -- interrupt callbacks -- */
	void (*transfer_ack_begin)(struct snd_pcm_substream *substream);
	void (*transfer_ack_end)(struct snd_pcm_substream *substream);

	/* -- timer -- */
	unsigned int timer_resolution;	/* timer resolution */

	/* -- DMA -- */           
	unsigned char *dma_area;	/* DMA area */
	dma_addr_t dma_addr;		/* physical bus address (not accessible from main CPU) */
	size_t dma_bytes;		/* size of DMA area */

	struct snd_dma_buffer *dma_buffer_p;	/* allocated buffer */

#if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE)
	/* -- OSS things -- */
	struct snd_pcm_oss_runtime oss;
#endif
};
]]>
            </programlisting>
          </informalexample>
	</para>

	<para>
	  For the operators (callbacks) of each sound driver, most of
	these records are supposed to be read-only.  Only the PCM
	middle-layer changes / updates them.  The exceptions are
	the hardware description (hw), interrupt callbacks
	(transfer_ack_xxx), DMA buffer information, and the private
	data.  Besides, if you use the standard buffer allocation
	method via <function>snd_pcm_lib_malloc_pages()</function>,
	you don't need to set the DMA buffer information by yourself.
	</para>

	<para>
	In the sections below, important records are explained.
	</para>

	<section id="pcm-interface-runtime-hw">
	<title>Hardware Description</title>
	<para>
	  The hardware descriptor (struct <structname>snd_pcm_hardware</structname>)
	contains the definitions of the fundamental hardware
	configuration.  Above all, you'll need to define this in
	<link linkend="pcm-interface-operators-open-callback"><citetitle>
	the open callback</citetitle></link>.
	Note that the runtime instance holds the copy of the
	descriptor, not the pointer to the existing descriptor.  That
	is, in the open callback, you can modify the copied descriptor
	(<constant>runtime-&gt;hw</constant>) as you need.  For example, if the maximum
	number of channels is 1 only on some chip models, you can
	still use the same hardware descriptor and change the
	channels_max later:
          <informalexample>
            <programlisting>
<![CDATA[
          struct snd_pcm_runtime *runtime = substream->runtime;
          ...
          runtime->hw = snd_mychip_playback_hw; /* common definition */
          if (chip->model == VERY_OLD_ONE)
                  runtime->hw.channels_max = 1;
]]>
            </programlisting>
          </informalexample>
	</para>

	<para>
	  Typically, you'll have a hardware descriptor as below:
          <informalexample>
            <programlisting>
<![CDATA[
  static struct snd_pcm_hardware snd_mychip_playback_hw = {
          .info = (SNDRV_PCM_INFO_MMAP |
                   SNDRV_PCM_INFO_INTERLEAVED |
                   SNDRV_PCM_INFO_BLOCK_TRANSFER |
                   SNDRV_PCM_INFO_MMAP_VALID),
          .formats =          SNDRV_PCM_FMTBIT_S16_LE,
          .rates =            SNDRV_PCM_RATE_8000_48000,
          .rate_min =         8000,
          .rate_max =         48000,
          .channels_min =     2,
          .channels_max =     2,
          .buffer_bytes_max = 32768,
          .period_bytes_min = 4096,
          .period_bytes_max = 32768,
          .periods_min =      1,
          .periods_max =      1024,
  };
]]>
            </programlisting>
          </informalexample>
        </para>

        <para>
	<itemizedlist>
	<listitem><para>
          The <structfield>info</structfield> field contains the type and
        capabilities of this pcm. The bit flags are defined in
        <filename>&lt;sound/asound.h&gt;</filename> as
        <constant>SNDRV_PCM_INFO_XXX</constant>. Here, at least, you
        have to specify whether the mmap is supported and which
        interleaved format is supported.
        When the is supported, add the
        <constant>SNDRV_PCM_INFO_MMAP</constant> flag here. When the
        hardware supports the interleaved or the non-interleaved
        formats, <constant>SNDRV_PCM_INFO_INTERLEAVED</constant> or
        <constant>SNDRV_PCM_INFO_NONINTERLEAVED</constant> flag must
        be set, respectively. If both are supported, you can set both,
        too. 
        </para>

        <para>
          In the above example, <constant>MMAP_VALID</constant> and
        <constant>BLOCK_TRANSFER</constant> are specified for the OSS mmap
        mode. Usually both are set. Of course,
        <constant>MMAP_VALID</constant> is set only if the mmap is
        really supported. 
        </para>

        <para>
          The other possible flags are
        <constant>SNDRV_PCM_INFO_PAUSE</constant> and
        <constant>SNDRV_PCM_INFO_RESUME</constant>. The
        <constant>PAUSE</constant> bit means that the pcm supports the
        <quote>pause</quote> operation, while the
        <constant>RESUME</constant> bit means that the pcm supports
        the full <quote>suspend/resume</quote> operation.
	If the <constant>PAUSE</constant> flag is set,
	the <structfield>trigger</structfield> callback below
        must handle the corresponding (pause push/release) commands.
	The suspend/resume trigger commands can be defined even without
	the <constant>RESUME</constant> flag.  See <link
	linkend="power-management"><citetitle>
	Power Management</citetitle></link> section for details.
        </para>

	<para>
	  When the PCM substreams can be synchronized (typically,
	synchronized start/stop of a playback and a capture streams),
	you can give <constant>SNDRV_PCM_INFO_SYNC_START</constant>,
	too.  In this case, you'll need to check the linked-list of
	PCM substreams in the trigger callback.  This will be
	described in the later section.
	</para>
	</listitem>

	<listitem>
        <para>
          <structfield>formats</structfield> field contains the bit-flags
        of supported formats (<constant>SNDRV_PCM_FMTBIT_XXX</constant>).
        If the hardware supports more than one format, give all or'ed
        bits.  In the example above, the signed 16bit little-endian
        format is specified.
        </para>
	</listitem>

	<listitem>
        <para>
        <structfield>rates</structfield> field contains the bit-flags of
        supported rates (<constant>SNDRV_PCM_RATE_XXX</constant>).
        When the chip supports continuous rates, pass
        <constant>CONTINUOUS</constant> bit additionally.
        The pre-defined rate bits are provided only for typical
	rates. If your chip supports unconventional rates, you need to add
        the <constant>KNOT</constant> bit and set up the hardware
        constraint manually (explained later).
        </para>
	</listitem>

	<listitem>
	<para>
	<structfield>rate_min</structfield> and
	<structfield>rate_max</structfield> define the minimum and
	maximum sample rate.  This should correspond somehow to
	<structfield>rates</structfield> bits.
	</para>
	</listitem>

	<listitem>
	<para>
	<structfield>channel_min</structfield> and
	<structfield>channel_max</structfield> 
	define, as you might already expected, the minimum and maximum
	number of channels.
	</para>
	</listitem>

	<listitem>
	<para>
	<structfield>buffer_bytes_max</structfield> defines the
	maximum buffer size in bytes.  There is no
	<structfield>buffer_bytes_min</structfield> field, since
	it can be calculated from the minimum period size and the
	minimum number of periods.
	Meanwhile, <structfield>period_bytes_min</structfield> and
	define the minimum and maximum size of the period in bytes.
	<structfield>periods_max</structfield> and
	<structfield>periods_min</structfield> define the maximum and
	minimum number of periods in the buffer.
        </para>

	<para>
	The <quote>period</quote> is a term that corresponds to
	a fragment in the OSS world. The period defines the size at
	which a PCM interrupt is generated. This size strongly
	depends on the hardware. 
	Generally, the smaller period size will give you more
	interrupts, that is, more controls. 
	In the case of capture, this size defines the input latency.
	On the other hand, the whole buffer size defines the
	output latency for the playback direction.
	</para>
	</listitem>

	<listitem>
	<para>
	There is also a field <structfield>fifo_size</structfield>.
	This specifies the size of the hardware FIFO, but currently it
	is neither used in the driver nor in the alsa-lib.  So, you
	can ignore this field.
	</para>
	</listitem>
	</itemizedlist>
	</para>
	</section>

	<section id="pcm-interface-runtime-config">
	<title>PCM Configurations</title>
	<para>
	Ok, let's go back again to the PCM runtime records.
	The most frequently referred records in the runtime instance are
	the PCM configurations.
	The PCM configurations are stored in the runtime instance
	after the application sends <type>hw_params</type> data via
	alsa-lib.  There are many fields copied from hw_params and
	sw_params structs.  For example,
	<structfield>format</structfield> holds the format type
	chosen by the application.  This field contains the enum value
	<constant>SNDRV_PCM_FORMAT_XXX</constant>.
	</para>

	<para>
	One thing to be noted is that the configured buffer and period
	sizes are stored in <quote>frames</quote> in the runtime.
        In the ALSA world, 1 frame = channels * samples-size.
	For conversion between frames and bytes, you can use the
	<function>frames_to_bytes()</function> and
          <function>bytes_to_frames()</function> helper functions. 
          <informalexample>
            <programlisting>
<![CDATA[
  period_bytes = frames_to_bytes(runtime, runtime->period_size);
]]>
            </programlisting>
          </informalexample>
        </para>

	<para>
	Also, many software parameters (sw_params) are
	stored in frames, too.  Please check the type of the field.
	<type>snd_pcm_uframes_t</type> is for the frames as unsigned
	integer while <type>snd_pcm_sframes_t</type> is for the frames
	as signed integer.
	</para>
	</section>

	<section id="pcm-interface-runtime-dma">
	<title>DMA Buffer Information</title>
	<para>
	The DMA buffer is defined by the following four fields,
	<structfield>dma_area</structfield>,
	<structfield>dma_addr</structfield>,
	<structfield>dma_bytes</structfield> and
	<structfield>dma_private</structfield>.
	The <structfield>dma_area</structfield> holds the buffer
	pointer (the logical address).  You can call
	<function>memcpy</function> from/to 
	this pointer.  Meanwhile, <structfield>dma_addr</structfield>
	holds the physical address of the buffer.  This field is
	specified only when the buffer is a linear buffer.
	<structfield>dma_bytes</structfield> holds the size of buffer
	in bytes.  <structfield>dma_private</structfield> is used for
	the ALSA DMA allocator.
	</para>

	<para>
	If you use a standard ALSA function,
	<function>snd_pcm_lib_malloc_pages()</function>, for
	allocating the buffer, these fields are set by the ALSA middle
	layer, and you should <emphasis>not</emphasis> change them by
	yourself.  You can read them but not write them.
	On the other hand, if you want to allocate the buffer by
	yourself, you'll need to manage it in hw_params callback.
	At least, <structfield>dma_bytes</structfield> is mandatory.
	<structfield>dma_area</structfield> is necessary when the
	buffer is mmapped.  If your driver doesn't support mmap, this
	field is not necessary.  <structfield>dma_addr</structfield>
	is also optional.  You can use
	<structfield>dma_private</structfield> as you like, too.
	</para>
	</section>

	<section id="pcm-interface-runtime-status">
	<title>Running Status</title>
	<para>
	The running status can be referred via <constant>runtime-&gt;status</constant>.
	This is the pointer to the struct <structname>snd_pcm_mmap_status</structname>
	record.  For example, you can get the current DMA hardware
	pointer via <constant>runtime-&gt;status-&gt;hw_ptr</constant>.
	</para>

	<para>
	The DMA application pointer can be referred via
	<constant>runtime-&gt;control</constant>, which points to the
	struct <structname>snd_pcm_mmap_control</structname> record.
	However, accessing directly to this value is not recommended.
	</para>
	</section>

	<section id="pcm-interface-runtime-private">
	<title>Private Data</title> 
	<para>
	You can allocate a record for the substream and store it in
	<constant>runtime-&gt;private_data</constant>.  Usually, this
	is done in
	<link linkend="pcm-interface-operators-open-callback"><citetitle>
	the open callback</citetitle></link>.
	Don't mix this with <constant>pcm-&gt;private_data</constant>.
	The <constant>pcm-&gt;private_data</constant> usually points to the
	chip instance assigned statically at the creation of PCM, while the 
	<constant>runtime-&gt;private_data</constant> points to a dynamic
	data structure created at the PCM open callback.

          <informalexample>
            <programlisting>
<![CDATA[
  static int snd_xxx_open(struct snd_pcm_substream *substream)
  {
          struct my_pcm_data *data;
          ....
          data = kmalloc(sizeof(*data), GFP_KERNEL);
          substream->runtime->private_data = data;
          ....
  }
]]>
            </programlisting>
          </informalexample>
        </para>

        <para>
          The allocated object must be released in
	<link linkend="pcm-interface-operators-open-callback"><citetitle>
	the close callback</citetitle></link>.
        </para>
	</section>

	<section id="pcm-interface-runtime-intr">
	<title>Interrupt Callbacks</title>
	<para>
	The field <structfield>transfer_ack_begin</structfield> and
	<structfield>transfer_ack_end</structfield> are called at
	the beginning and at the end of
	<function>snd_pcm_period_elapsed()</function>, respectively. 
	</para>
	</section>

    </section>

    <section id="pcm-interface-operators">
      <title>Operators</title>
      <para>
        OK, now let me give details about each pcm callback
      (<parameter>ops</parameter>). In general, every callback must
      return 0 if successful, or a negative error number
      such as <constant>-EINVAL</constant>. To choose an appropriate
      error number, it is advised to check what value other parts of
      the kernel return when the same kind of request fails.
      </para>

      <para>
        The callback function takes at least the argument with
        <structname>snd_pcm_substream</structname> pointer. To retrieve
        the chip record from the given substream instance, you can use the
        following macro. 

        <informalexample>
          <programlisting>
<![CDATA[
  int xxx() {
          struct mychip *chip = snd_pcm_substream_chip(substream);
          ....
  }
]]>
          </programlisting>
        </informalexample>

	The macro reads <constant>substream-&gt;private_data</constant>,
	which is a copy of <constant>pcm-&gt;private_data</constant>.
	You can override the former if you need to assign different data
	records per PCM substream.  For example, the cmi8330 driver assigns
	different private_data for playback and capture directions,
	because it uses two different codecs (SB- and AD-compatible) for
	different directions.
      </para>

      <section id="pcm-interface-operators-open-callback">
        <title>open callback</title>
        <para>
          <informalexample>
            <programlisting>
<![CDATA[
  static int snd_xxx_open(struct snd_pcm_substream *substream);
]]>
            </programlisting>
          </informalexample>

          This is called when a pcm substream is opened.
        </para>

        <para>
          At least, here you have to initialize the runtime-&gt;hw
          record. Typically, this is done by like this: 

          <informalexample>
            <programlisting>
<![CDATA[
  static int snd_xxx_open(struct snd_pcm_substream *substream)
  {
          struct mychip *chip = snd_pcm_substream_chip(substream);
          struct snd_pcm_runtime *runtime = substream->runtime;

          runtime->hw = snd_mychip_playback_hw;
          return 0;
  }
]]>
            </programlisting>
          </informalexample>

          where <parameter>snd_mychip_playback_hw</parameter> is the
          pre-defined hardware description.
	</para>

	<para>
	You can allocate a private data in this callback, as described
	in <link linkend="pcm-interface-runtime-private"><citetitle>
	Private Data</citetitle></link> section.
	</para>

	<para>
	If the hardware configuration needs more constraints, set the
	hardware constraints here, too.
	See <link linkend="pcm-interface-constraints"><citetitle>
	Constraints</citetitle></link> for more details.
	</para>
      </section>

      <section id="pcm-interface-operators-close-callback">
        <title>close callback</title>
        <para>
          <informalexample>
            <programlisting>
<![CDATA[
  static int snd_xxx_close(struct snd_pcm_substream *substream);
]]>
            </programlisting>
          </informalexample>

          Obviously, this is called when a pcm substream is closed.
        </para>

        <para>
          Any private instance for a pcm substream allocated in the
          open callback will be released here. 

          <informalexample>
            <programlisting>
<![CDATA[
  static int snd_xxx_close(struct snd_pcm_substream *substream)
  {
          ....
          kfree(substream->runtime->private_data);
          ....
  }
]]>
            </programlisting>
          </informalexample>
        </para>
      </section>

      <section id="pcm-interface-operators-ioctl-callback">
        <title>ioctl callback</title>
        <para>
          This is used for any special call to pcm ioctls. But
        usually you can pass a generic ioctl callback, 
        <function>snd_pcm_lib_ioctl</function>.
        </para>
      </section>

      <section id="pcm-interface-operators-hw-params-callback">
        <title>hw_params callback</title>
        <para>
          <informalexample>
            <programlisting>
<![CDATA[
  static int snd_xxx_hw_params(struct snd_pcm_substream *substream,
                               struct snd_pcm_hw_params *hw_params);
]]>
            </programlisting>
          </informalexample>
        </para>

        <para>
          This is called when the hardware parameter
        (<structfield>hw_params</structfield>) is set
        up by the application, 
        that is, once when the buffer size, the period size, the
        format, etc. are defined for the pcm substream. 
        </para>

        <para>
          Many hardware setups should be done in this callback,
        including the allocation of buffers. 
        </para>

        <para>
          Parameters to be initialized are retrieved by
          <function>params_xxx()</function> macros. To allocate
          buffer, you can call a helper function, 

          <informalexample>
            <programlisting>
<![CDATA[
  snd_pcm_lib_malloc_pages(substream, params_buffer_bytes(hw_params));
]]>
            </programlisting>
          </informalexample>

          <function>snd_pcm_lib_malloc_pages()</function> is available
	  only when the DMA buffers have been pre-allocated.
	  See the section <link
	  linkend="buffer-and-memory-buffer-types"><citetitle>
	  Buffer Types</citetitle></link> for more details.
        </para>

        <para>
          Note that this and <structfield>prepare</structfield> callbacks
        may be called multiple times per initialization.
        For example, the OSS emulation may
        call these callbacks at each change via its ioctl. 
        </para>

        <para>
          Thus, you need to be careful not to allocate the same buffers
        many times, which will lead to memory leaks!  Calling the
        helper function above many times is OK. It will release the
        previous buffer automatically when it was already allocated. 
        </para>

        <para>
          Another note is that this callback is non-atomic
        (schedulable). This is important, because the
        <structfield>trigger</structfield> callback 
        is atomic (non-schedulable). That is, mutexes or any
        schedule-related functions are not available in
        <structfield>trigger</structfield> callback.
	Please see the subsection
	<link linkend="pcm-interface-atomicity"><citetitle>
	Atomicity</citetitle></link> for details.
        </para>
      </section>

      <section id="pcm-interface-operators-hw-free-callback">
        <title>hw_free callback</title>
        <para>
          <informalexample>
            <programlisting>
<![CDATA[
  static int snd_xxx_hw_free(struct snd_pcm_substream *substream);
]]>
            </programlisting>
          </informalexample>
        </para>

        <para>
          This is called to release the resources allocated via
          <structfield>hw_params</structfield>. For example, releasing the
          buffer via 
          <function>snd_pcm_lib_malloc_pages()</function> is done by
          calling the following: 

          <informalexample>
            <programlisting>
<![CDATA[
  snd_pcm_lib_free_pages(substream);
]]>
            </programlisting>
          </informalexample>
        </para>

        <para>
          This function is always called before the close callback is called.
          Also, the callback may be called multiple times, too.
          Keep track whether the resource was already released. 
        </para>
      </section>

      <section id="pcm-interface-operators-prepare-callback">
       <title>prepare callback</title>
        <para>
          <informalexample>
            <programlisting>
<![CDATA[
  static int snd_xxx_prepare(struct snd_pcm_substream *substream);
]]>
            </programlisting>
          </informalexample>
        </para>

        <para>
          This callback is called when the pcm is
        <quote>prepared</quote>. You can set the format type, sample
        rate, etc. here. The difference from
        <structfield>hw_params</structfield> is that the 
        <structfield>prepare</structfield> callback will be called each
        time 
        <function>snd_pcm_prepare()</function> is called, i.e. when
        recovering after underruns, etc. 
        </para>

        <para>
	Note that this callback is now non-atomic.
	You can use schedule-related functions safely in this callback.
        </para>

        <para>
          In this and the following callbacks, you can refer to the
        values via the runtime record,
        substream-&gt;runtime.
        For example, to get the current
        rate, format or channels, access to
        runtime-&gt;rate,
        runtime-&gt;format or
        runtime-&gt;channels, respectively. 
        The physical address of the allocated buffer is set to
	runtime-&gt;dma_area.  The buffer and period sizes are
	in runtime-&gt;buffer_size and runtime-&gt;period_size,
	respectively.
        </para>

        <para>
          Be careful that this callback will be called many times at
        each setup, too. 
        </para>
      </section>

      <section id="pcm-interface-operators-trigger-callback">
        <title>trigger callback</title>
        <para>
          <informalexample>
            <programlisting>
<![CDATA[
  static int snd_xxx_trigger(struct snd_pcm_substream *substream, int cmd);
]]>
            </programlisting>
          </informalexample>

          This is called when the pcm is started, stopped or paused.
        </para>

        <para>
          Which action is specified in the second argument,
          <constant>SNDRV_PCM_TRIGGER_XXX</constant> in
          <filename>&lt;sound/pcm.h&gt;</filename>. At least,
          the <constant>START</constant> and <constant>STOP</constant>
          commands must be defined in this callback. 

          <informalexample>
            <programlisting>
<![CDATA[
  switch (cmd) {
  case SNDRV_PCM_TRIGGER_START:
          /* do something to start the PCM engine */
          break;
  case SNDRV_PCM_TRIGGER_STOP:
          /* do something to stop the PCM engine */
          break;
  default:
          return -EINVAL;
  }
]]>
            </programlisting>
          </informalexample>
        </para>

        <para>
          When the pcm supports the pause operation (given in the info
        field of the hardware table), the <constant>PAUSE_PUSE</constant>
        and <constant>PAUSE_RELEASE</constant> commands must be
        handled here, too. The former is the command to pause the pcm,
        and the latter to restart the pcm again. 
        </para>

        <para>
          When the pcm supports the suspend/resume operation,
	regardless of full or partial suspend/resume support,
        the <constant>SUSPEND</constant> and <constant>RESUME</constant>
        commands must be handled, too.
        These commands are issued when the power-management status is
        changed.  Obviously, the <constant>SUSPEND</constant> and
        <constant>RESUME</constant> commands
        suspend and resume the pcm substream, and usually, they
        are identical to the <constant>STOP</constant> and
        <constant>START</constant> commands, respectively.
	  See the <link linkend="power-management"><citetitle>
	Power Management</citetitle></link> section for details.
        </para>

        <para>
          As mentioned, this callback is atomic.  You cannot call
	  functions which may sleep.
	  The trigger callback should be as minimal as possible,
	  just really triggering the DMA.  The other stuff should be
	  initialized hw_params and prepare callbacks properly
	  beforehand.
        </para>
      </section>

      <section id="pcm-interface-operators-pointer-callback">
        <title>pointer callback</title>
        <para>
          <informalexample>
            <programlisting>
<![CDATA[
  static snd_pcm_uframes_t snd_xxx_pointer(struct snd_pcm_substream *substream)
]]>
            </programlisting>
          </informalexample>

          This callback is called when the PCM middle layer inquires
        the current hardware position on the buffer. The position must
        be returned in frames,
        ranging from 0 to buffer_size - 1.
        </para>

        <para>
          This is called usually from the buffer-update routine in the
        pcm middle layer, which is invoked when
        <function>snd_pcm_period_elapsed()</function> is called in the
        interrupt routine. Then the pcm middle layer updates the
        position and calculates the available space, and wakes up the
        sleeping poll threads, etc. 
        </para>

        <para>
          This callback is also atomic.
        </para>
      </section>

      <section id="pcm-interface-operators-copy-silence">
        <title>copy and silence callbacks</title>
        <para>
          These callbacks are not mandatory, and can be omitted in
        most cases. These callbacks are used when the hardware buffer
        cannot be in the normal memory space. Some chips have their
        own buffer on the hardware which is not mappable. In such a
        case, you have to transfer the data manually from the memory
        buffer to the hardware buffer. Or, if the buffer is
        non-contiguous on both physical and virtual memory spaces,
        these callbacks must be defined, too. 
        </para>

        <para>
          If these two callbacks are defined, copy and set-silence
        operations are done by them. The detailed will be described in
        the later section <link
        linkend="buffer-and-memory"><citetitle>Buffer and Memory
        Management</citetitle></link>. 
        </para>
      </section>

      <section id="pcm-interface-operators-ack">
        <title>ack callback</title>
        <para>
          This callback is also not mandatory. This callback is called
        when the appl_ptr is updated in read or write operations.
        Some drivers like emu10k1-fx and cs46xx need to track the
	current appl_ptr for the internal buffer, and this callback
	is useful only for such a purpose.
	</para>
	<para>
	  This callback is atomic.
	</para>
      </section>

      <section id="pcm-interface-operators-page-callback">
        <title>page callback</title>

        <para>
          This callback is optional too. This callback is used
        mainly for non-contiguous buffers. The mmap calls this
        callback to get the page address. Some examples will be
        explained in the later section <link
        linkend="buffer-and-memory"><citetitle>Buffer and Memory
        Management</citetitle></link>, too. 
        </para>
      </section>
    </section>

    <section id="pcm-interface-interrupt-handler">
      <title>Interrupt Handler</title>
      <para>
        The rest of pcm stuff is the PCM interrupt handler. The
      role of PCM interrupt handler in the sound driver is to update
      the buffer position and to tell the PCM middle layer when the
      buffer position goes across the prescribed period size. To
      inform this, call the <function>snd_pcm_period_elapsed()</function>
      function. 
      </para>

      <para>
        There are several types of sound chips to generate the interrupts.
      </para>

      <section id="pcm-interface-interrupt-handler-boundary">
        <title>Interrupts at the period (fragment) boundary</title>
        <para>
          This is the most frequently found type:  the hardware
        generates an interrupt at each period boundary.
	In this case, you can call
        <function>snd_pcm_period_elapsed()</function> at each 
        interrupt. 
        </para>

        <para>
          <function>snd_pcm_period_elapsed()</function> takes the
        substream pointer as its argument. Thus, you need to keep the
        substream pointer accessible from the chip instance. For
        example, define substream field in the chip record to hold the
        current running substream pointer, and set the pointer value
        at open callback (and reset at close callback). 
        </para>

        <para>
          If you acquire a spinlock in the interrupt handler, and the
        lock is used in other pcm callbacks, too, then you have to
        release the lock before calling
        <function>snd_pcm_period_elapsed()</function>, because
        <function>snd_pcm_period_elapsed()</function> calls other pcm
        callbacks inside. 
        </para>

        <para>
          Typical code would be like:

          <example>
	    <title>Interrupt Handler Case #1</title>
            <programlisting>
<![CDATA[
  static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
  {
          struct mychip *chip = dev_id;
          spin_lock(&chip->lock);
          ....
          if (pcm_irq_invoked(chip)) {
                  /* call updater, unlock before it */
                  spin_unlock(&chip->lock);
                  snd_pcm_period_elapsed(chip->substream);
                  spin_lock(&chip->lock);
                  /* acknowledge the interrupt if necessary */
          }
          ....
          spin_unlock(&chip->lock);
          return IRQ_HANDLED;
  }
]]>
            </programlisting>
          </example>
        </para>
      </section>

      <section id="pcm-interface-interrupt-handler-timer">
        <title>High frequency timer interrupts</title>
        <para>
	This happense when the hardware doesn't generate interrupts
        at the period boundary but issues timer interrupts at a fixed
        timer rate (e.g. es1968 or ymfpci drivers). 
        In this case, you need to check the current hardware
        position and accumulate the processed sample length at each
        interrupt.  When the accumulated size exceeds the period
        size, call 
        <function>snd_pcm_period_elapsed()</function> and reset the
        accumulator. 
        </para>

        <para>
          Typical code would be like the following.

          <example>
	    <title>Interrupt Handler Case #2</title>
            <programlisting>
<![CDATA[
  static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
  {
          struct mychip *chip = dev_id;
          spin_lock(&chip->lock);
          ....
          if (pcm_irq_invoked(chip)) {
                  unsigned int last_ptr, size;
                  /* get the current hardware pointer (in frames) */
                  last_ptr = get_hw_ptr(chip);
                  /* calculate the processed frames since the
                   * last update
                   */
                  if (last_ptr < chip->last_ptr)
                          size = runtime->buffer_size + last_ptr 
                                   - chip->last_ptr; 
                  else
                          size = last_ptr - chip->last_ptr;
                  /* remember the last updated point */
                  chip->last_ptr = last_ptr;
                  /* accumulate the size */
                  chip->size += size;
                  /* over the period boundary? */
                  if (chip->size >= runtime->period_size) {
                          /* reset the accumulator */
                          chip->size %= runtime->period_size;
                          /* call updater */
                          spin_unlock(&chip->lock);
                          snd_pcm_period_elapsed(substream);
                          spin_lock(&chip->lock);
                  }
                  /* acknowledge the interrupt if necessary */
          }
          ....
          spin_unlock(&chip->lock);
          return IRQ_HANDLED;
  }
]]>
            </programlisting>
          </example>
        </para>
      </section>

      <section id="pcm-interface-interrupt-handler-both">
        <title>On calling <function>snd_pcm_period_elapsed()</function></title>
        <para>
          In both cases, even if more than one period are elapsed, you
        don't have to call
        <function>snd_pcm_period_elapsed()</function> many times. Call
        only once. And the pcm layer will check the current hardware
        pointer and update to the latest status. 
        </para>
      </section>
    </section>

    <section id="pcm-interface-atomicity">
      <title>Atomicity</title>
      <para>
      One of the most important (and thus difficult to debug) problems
      in kernel programming are race conditions.
      In the Linux kernel, they are usually avoided via spin-locks, mutexes
      or semaphores.  In general, if a race condition can happen
      in an interrupt handler, it has to be managed atomically, and you
      have to use a spinlock to protect the critical session. If the
      critical section is not in interrupt handler code and
      if taking a relatively long time to execute is acceptable, you
      should use mutexes or semaphores instead.
      </para>

      <para>
      As already seen, some pcm callbacks are atomic and some are
      not.  For example, the <parameter>hw_params</parameter> callback is
      non-atomic, while <parameter>trigger</parameter> callback is
      atomic.  This means, the latter is called already in a spinlock
      held by the PCM middle layer. Please take this atomicity into
      account when you choose a locking scheme in the callbacks.
      </para>

      <para>
      In the atomic callbacks, you cannot use functions which may call
      <function>schedule</function> or go to
      <function>sleep</function>.  Semaphores and mutexes can sleep,
      and hence they cannot be used inside the atomic callbacks
      (e.g. <parameter>trigger</parameter> callback).
      To implement some delay in such a callback, please use
      <function>udelay()</function> or <function>mdelay()</function>.
      </para>

      <para>
      All three atomic callbacks (trigger, pointer, and ack) are
      called with local interrupts disabled.
      </para>

    </section>
    <section id="pcm-interface-constraints">
      <title>Constraints</title>
      <para>
        If your chip supports unconventional sample rates, or only the
      limited samples, you need to set a constraint for the
      condition. 
      </para>

      <para>
        For example, in order to restrict the sample rates in the some
        supported values, use
	<function>snd_pcm_hw_constraint_list()</function>.
	You need to call this function in the open callback.

        <example>
	  <title>Example of Hardware Constraints</title>
          <programlisting>
<![CDATA[
  static unsigned int rates[] =
          {4000, 10000, 22050, 44100};
  static struct snd_pcm_hw_constraint_list constraints_rates = {
          .count = ARRAY_SIZE(rates),
          .list = rates,
          .mask = 0,
  };

  static int snd_mychip_pcm_open(struct snd_pcm_substream *substream)
  {
          int err;
          ....
          err = snd_pcm_hw_constraint_list(substream->runtime, 0,
                                           SNDRV_PCM_HW_PARAM_RATE,
                                           &constraints_rates);
          if (err < 0)
                  return err;
          ....
  }
]]>
          </programlisting>
        </example>
      </para>

      <para>
        There are many different constraints.
        Look at <filename>sound/pcm.h</filename> for a complete list.
        You can even define your own constraint rules.
        For example, let's suppose my_chip can manage a substream of 1 channel
        if and only if the format is S16_LE, otherwise it supports any format
        specified in the <structname>snd_pcm_hardware</structname> structure (or in any
        other constraint_list). You can build a rule like this:

        <example>
	  <title>Example of Hardware Constraints for Channels</title>
	  <programlisting>
<![CDATA[
  static int hw_rule_format_by_channels(struct snd_pcm_hw_params *params,
                                        struct snd_pcm_hw_rule *rule)
  {
          struct snd_interval *c = hw_param_interval(params,
                SNDRV_PCM_HW_PARAM_CHANNELS);
          struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
          struct snd_mask fmt;

          snd_mask_any(&fmt);    /* Init the struct */
          if (c->min < 2) {
                  fmt.bits[0] &= SNDRV_PCM_FMTBIT_S16_LE;
                  return snd_mask_refine(f, &fmt);
          }
          return 0;
  }
]]>
          </programlisting>
        </example>
      </para>
 
      <para>
        Then you need to call this function to add your rule:

       <informalexample>
	 <programlisting>
<![CDATA[
  snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_CHANNELS,
                      hw_rule_channels_by_format, 0, SNDRV_PCM_HW_PARAM_FORMAT,
                      -1);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        The rule function is called when an application sets the number of
        channels. But an application can set the format before the number of
        channels. Thus you also need to define the inverse rule:

       <example>
	 <title>Example of Hardware Constraints for Channels</title>
	 <programlisting>
<![CDATA[
  static int hw_rule_channels_by_format(struct snd_pcm_hw_params *params,
                                        struct snd_pcm_hw_rule *rule)
  {
          struct snd_interval *c = hw_param_interval(params,
                        SNDRV_PCM_HW_PARAM_CHANNELS);
          struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
          struct snd_interval ch;

          snd_interval_any(&ch);
          if (f->bits[0] == SNDRV_PCM_FMTBIT_S16_LE) {
                  ch.min = ch.max = 1;
                  ch.integer = 1;
                  return snd_interval_refine(c, &ch);
          }
          return 0;
  }
]]>
          </programlisting>
        </example>
      </para>

      <para>
      ...and in the open callback:
       <informalexample>
	 <programlisting>
<![CDATA[
  snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_FORMAT,
                      hw_rule_format_by_channels, 0, SNDRV_PCM_HW_PARAM_CHANNELS,
                      -1);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        I won't give more details here, rather I
        would like to say, <quote>Luke, use the source.</quote>
      </para>
    </section>

  </chapter>


<!-- ****************************************************** -->
<!-- Control Interface  -->
<!-- ****************************************************** -->
  <chapter id="control-interface">
    <title>Control Interface</title>

    <section id="control-interface-general">
      <title>General</title>
      <para>
        The control interface is used widely for many switches,
      sliders, etc. which are accessed from user-space. Its most
      important use is the mixer interface. In other words, since ALSA
      0.9.x, all the mixer stuff is implemented on the control kernel API.
      </para>

      <para>
        ALSA has a well-defined AC97 control module. If your chip
      supports only the AC97 and nothing else, you can skip this
      section. 
      </para>

      <para>
        The control API is defined in
      <filename>&lt;sound/control.h&gt;</filename>.
      Include this file if you want to add your own controls.
      </para>
    </section>

    <section id="control-interface-definition">
      <title>Definition of Controls</title>
      <para>
        To create a new control, you need to define the
	following three
      callbacks: <structfield>info</structfield>,
      <structfield>get</structfield> and
      <structfield>put</structfield>. Then, define a
      struct <structname>snd_kcontrol_new</structname> record, such as: 

        <example>
	  <title>Definition of a Control</title>
          <programlisting>
<![CDATA[
  static struct snd_kcontrol_new my_control __devinitdata = {
          .iface = SNDRV_CTL_ELEM_IFACE_MIXER,
          .name = "PCM Playback Switch",
          .index = 0,
          .access = SNDRV_CTL_ELEM_ACCESS_READWRITE,
          .private_value = 0xffff,
          .info = my_control_info,
          .get = my_control_get,
          .put = my_control_put
  };
]]>
          </programlisting>
        </example>
      </para>

      <para>
        Most likely the control is created via
      <function>snd_ctl_new1()</function>, and in such a case, you can
      add the <parameter>__devinitdata</parameter> prefix to the
      definition as above. 
      </para>

      <para>
        The <structfield>iface</structfield> field specifies the control
      type, <constant>SNDRV_CTL_ELEM_IFACE_XXX</constant>, which
      is usually <constant>MIXER</constant>.
      Use <constant>CARD</constant> for global controls that are not
      logically part of the mixer.
      If the control is closely associated with some specific device on
      the sound card, use <constant>HWDEP</constant>,
      <constant>PCM</constant>, <constant>RAWMIDI</constant>,
      <constant>TIMER</constant>, or <constant>SEQUENCER</constant>, and
      specify the device number with the
      <structfield>device</structfield> and
      <structfield>subdevice</structfield> fields.
      </para>

      <para>
        The <structfield>name</structfield> is the name identifier
      string. Since ALSA 0.9.x, the control name is very important,
      because its role is classified from its name. There are
      pre-defined standard control names. The details are described in
      the <link linkend="control-interface-control-names"><citetitle>
      Control Names</citetitle></link> subsection.
      </para>

      <para>
        The <structfield>index</structfield> field holds the index number
      of this control. If there are several different controls with
      the same name, they can be distinguished by the index
      number. This is the case when 
      several codecs exist on the card. If the index is zero, you can
      omit the definition above. 
      </para>

      <para>
        The <structfield>access</structfield> field contains the access
      type of this control. Give the combination of bit masks,
      <constant>SNDRV_CTL_ELEM_ACCESS_XXX</constant>, there.
      The details will be explained in
      the <link linkend="control-interface-access-flags"><citetitle>
      Access Flags</citetitle></link> subsection.
      </para>

      <para>
        The <structfield>private_value</structfield> field contains
      an arbitrary long integer value for this record. When using
      the generic <structfield>info</structfield>,
      <structfield>get</structfield> and
      <structfield>put</structfield> callbacks, you can pass a value 
      through this field. If several small numbers are necessary, you can
      combine them in bitwise. Or, it's possible to give a pointer
      (casted to unsigned long) of some record to this field, too. 
      </para>

      <para>
      The <structfield>tlv</structfield> field can be used to provide
      metadata about the control; see the
      <link linkend="control-interface-tlv">
      <citetitle>Metadata</citetitle></link> subsection.
      </para>

      <para>
        The other three are
	<link linkend="control-interface-callbacks"><citetitle>
	callback functions</citetitle></link>.
      </para>
    </section>

    <section id="control-interface-control-names">
      <title>Control Names</title>
      <para>
        There are some standards to define the control names. A
      control is usually defined from the three parts as
      <quote>SOURCE DIRECTION FUNCTION</quote>. 
      </para>

      <para>
        The first, <constant>SOURCE</constant>, specifies the source
      of the control, and is a string such as <quote>Master</quote>,
      <quote>PCM</quote>, <quote>CD</quote> and
      <quote>Line</quote>. There are many pre-defined sources. 
      </para>

      <para>
        The second, <constant>DIRECTION</constant>, is one of the
      following strings according to the direction of the control:
      <quote>Playback</quote>, <quote>Capture</quote>, <quote>Bypass
      Playback</quote> and <quote>Bypass Capture</quote>. Or, it can
      be omitted, meaning both playback and capture directions. 
      </para>

      <para>
        The third, <constant>FUNCTION</constant>, is one of the
      following strings according to the function of the control:
      <quote>Switch</quote>, <quote>Volume</quote> and
      <quote>Route</quote>. 
      </para>

      <para>
        The example of control names are, thus, <quote>Master Capture
      Switch</quote> or <quote>PCM Playback Volume</quote>. 
      </para>

      <para>
        There are some exceptions:
      </para>

      <section id="control-interface-control-names-global">
        <title>Global capture and playback</title>
        <para>
          <quote>Capture Source</quote>, <quote>Capture Switch</quote>
        and <quote>Capture Volume</quote> are used for the global
        capture (input) source, switch and volume. Similarly,
        <quote>Playback Switch</quote> and <quote>Playback
        Volume</quote> are used for the global output gain switch and
        volume. 
        </para>
      </section>

      <section id="control-interface-control-names-tone">
        <title>Tone-controls</title>
        <para>
          tone-control switch and volumes are specified like
        <quote>Tone Control - XXX</quote>, e.g. <quote>Tone Control -
        Switch</quote>, <quote>Tone Control - Bass</quote>,
        <quote>Tone Control - Center</quote>.  
        </para>
      </section>

      <section id="control-interface-control-names-3d">
        <title>3D controls</title>
        <para>
          3D-control switches and volumes are specified like <quote>3D
        Control - XXX</quote>, e.g. <quote>3D Control -
        Switch</quote>, <quote>3D Control - Center</quote>, <quote>3D
        Control - Space</quote>. 
        </para>
      </section>

      <section id="control-interface-control-names-mic">
        <title>Mic boost</title>
        <para>
          Mic-boost switch is set as <quote>Mic Boost</quote> or
        <quote>Mic Boost (6dB)</quote>. 
        </para>

        <para>
          More precise information can be found in
        <filename>Documentation/sound/alsa/ControlNames.txt</filename>.
        </para>
      </section>
    </section>

    <section id="control-interface-access-flags">
      <title>Access Flags</title>

      <para>
      The access flag is the bitmask which specifies the access type
      of the given control.  The default access type is
      <constant>SNDRV_CTL_ELEM_ACCESS_READWRITE</constant>, 
      which means both read and write are allowed to this control.
      When the access flag is omitted (i.e. = 0), it is
      considered as <constant>READWRITE</constant> access as default. 
      </para>

      <para>
      When the control is read-only, pass
      <constant>SNDRV_CTL_ELEM_ACCESS_READ</constant> instead.
      In this case, you don't have to define
      the <structfield>put</structfield> callback.
      Similarly, when the control is write-only (although it's a rare
      case), you can use the <constant>WRITE</constant> flag instead, and
      you don't need the <structfield>get</structfield> callback.
      </para>

      <para>
      If the control value changes frequently (e.g. the VU meter),
      <constant>VOLATILE</constant> flag should be given.  This means
      that the control may be changed without
      <link linkend="control-interface-change-notification"><citetitle>
      notification</citetitle></link>. Applications should poll such
      a control constantly.
      </para>

      <para>
      When the control is inactive, set
      the <constant>INACTIVE</constant> flag, too.
      There are <constant>LOCK</constant> and
      <constant>OWNER</constant> flags to change the write
      permissions.
      </para>

    </section>

    <section id="control-interface-callbacks">
      <title>Callbacks</title>

      <section id="control-interface-callbacks-info">
        <title>info callback</title>
        <para>
          The <structfield>info</structfield> callback is used to get
        detailed information on this control. This must store the
        values of the given struct <structname>snd_ctl_elem_info</structname>
        object. For example, for a boolean control with a single
        element: 

          <example>
	    <title>Example of info callback</title>
            <programlisting>
<![CDATA[
  static int snd_myctl_mono_info(struct snd_kcontrol *kcontrol,
                          struct snd_ctl_elem_info *uinfo)
  {
          uinfo->type = SNDRV_CTL_ELEM_TYPE_BOOLEAN;
          uinfo->count = 1;
          uinfo->value.integer.min = 0;
          uinfo->value.integer.max = 1;
          return 0;
  }
]]>
            </programlisting>
          </example>
        </para>

        <para>
          The <structfield>type</structfield> field specifies the type
        of the control. There are <constant>BOOLEAN</constant>,
        <constant>INTEGER</constant>, <constant>ENUMERATED</constant>,
        <constant>BYTES</constant>, <constant>IEC958</constant> and
        <constant>INTEGER64</constant>. The
        <structfield>count</structfield> field specifies the 
        number of elements in this control. For example, a stereo
        volume would have count = 2. The
        <structfield>value</structfield> field is a union, and 
        the values stored are depending on the type. The boolean and
        integer types are identical. 
        </para>

        <para>
          The enumerated type is a bit different from others.  You'll
          need to set the string for the currently given item index. 

          <informalexample>
            <programlisting>
<![CDATA[
  static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol,
                          struct snd_ctl_elem_info *uinfo)
  {
          static char *texts[4] = {
                  "First", "Second", "Third", "Fourth"
          };
          uinfo->type = SNDRV_CTL_ELEM_TYPE_ENUMERATED;
          uinfo->count = 1;
          uinfo->value.enumerated.items = 4;
          if (uinfo->value.enumerated.item > 3)
                  uinfo->value.enumerated.item = 3;
          strcpy(uinfo->value.enumerated.name,
                 texts[uinfo->value.enumerated.item]);
          return 0;
  }
]]>
            </programlisting>
          </informalexample>
        </para>

        <para>
	  Some common info callbacks are available for your convenience:
	<function>snd_ctl_boolean_mono_info()</function> and
	<function>snd_ctl_boolean_stereo_info()</function>.
	Obviously, the former is an info callback for a mono channel
	boolean item, just like <function>snd_myctl_mono_info</function>
	above, and the latter is for a stereo channel boolean item.
	</para>

      </section>

      <section id="control-interface-callbacks-get">
        <title>get callback</title>

        <para>
          This callback is used to read the current value of the
        control and to return to user-space. 
        </para>

        <para>
          For example,

          <example>
	    <title>Example of get callback</title>
            <programlisting>
<![CDATA[
  static int snd_myctl_get(struct snd_kcontrol *kcontrol,
                           struct snd_ctl_elem_value *ucontrol)
  {
          struct mychip *chip = snd_kcontrol_chip(kcontrol);
          ucontrol->value.integer.value[0] = get_some_value(chip);
          return 0;
  }
]]>
            </programlisting>
          </example>
        </para>

        <para>
	The <structfield>value</structfield> field depends on 
        the type of control as well as on the info callback.  For example,
	the sb driver uses this field to store the register offset,
        the bit-shift and the bit-mask.  The
        <structfield>private_value</structfield> field is set as follows:
          <informalexample>
            <programlisting>
<![CDATA[
  .private_value = reg | (shift << 16) | (mask << 24)
]]>
            </programlisting>
          </informalexample>
	and is retrieved in callbacks like
          <informalexample>
            <programlisting>
<![CDATA[
  static int snd_sbmixer_get_single(struct snd_kcontrol *kcontrol,
                                    struct snd_ctl_elem_value *ucontrol)
  {
          int reg = kcontrol->private_value & 0xff;
          int shift = (kcontrol->private_value >> 16) & 0xff;
          int mask = (kcontrol->private_value >> 24) & 0xff;
          ....
  }
]]>
            </programlisting>
          </informalexample>
	</para>

	<para>
	In the <structfield>get</structfield> callback,
	you have to fill all the elements if the
        control has more than one elements,
        i.e. <structfield>count</structfield> &gt; 1.
	In the example above, we filled only one element
        (<structfield>value.integer.value[0]</structfield>) since it's
        assumed as <structfield>count</structfield> = 1.
        </para>
      </section>

      <section id="control-interface-callbacks-put">
        <title>put callback</title>

        <para>
          This callback is used to write a value from user-space.
        </para>

        <para>
          For example,

          <example>
	    <title>Example of put callback</title>
            <programlisting>
<![CDATA[
  static int snd_myctl_put(struct snd_kcontrol *kcontrol,
                           struct snd_ctl_elem_value *ucontrol)
  {
          struct mychip *chip = snd_kcontrol_chip(kcontrol);
          int changed = 0;
          if (chip->current_value !=
               ucontrol->value.integer.value[0]) {
                  change_current_value(chip,
                              ucontrol->value.integer.value[0]);
                  changed = 1;
          }
          return changed;
  }
]]>
            </programlisting>
          </example>

          As seen above, you have to return 1 if the value is
        changed. If the value is not changed, return 0 instead. 
	If any fatal error happens, return a negative error code as
        usual.
        </para>

        <para>
	As in the <structfield>get</structfield> callback,
	when the control has more than one elements,
	all elements must be evaluated in this callback, too.
        </para>
      </section>

      <section id="control-interface-callbacks-all">
        <title>Callbacks are not atomic</title>
        <para>
          All these three callbacks are basically not atomic.
        </para>
      </section>
    </section>

    <section id="control-interface-constructor">
      <title>Constructor</title>
      <para>
        When everything is ready, finally we can create a new
      control. To create a control, there are two functions to be
      called, <function>snd_ctl_new1()</function> and
      <function>snd_ctl_add()</function>. 
      </para>

      <para>
        In the simplest way, you can do like this:

        <informalexample>
          <programlisting>
<![CDATA[
  err = snd_ctl_add(card, snd_ctl_new1(&my_control, chip));
  if (err < 0)
          return err;
]]>
          </programlisting>
        </informalexample>

        where <parameter>my_control</parameter> is the
      struct <structname>snd_kcontrol_new</structname> object defined above, and chip
      is the object pointer to be passed to
      kcontrol-&gt;private_data 
      which can be referred to in callbacks. 
      </para>

      <para>
        <function>snd_ctl_new1()</function> allocates a new
      <structname>snd_kcontrol</structname> instance (that's why the definition
      of <parameter>my_control</parameter> can be with
      the <parameter>__devinitdata</parameter> 
      prefix), and <function>snd_ctl_add</function> assigns the given
      control component to the card. 
      </para>
    </section>

    <section id="control-interface-change-notification">
      <title>Change Notification</title>
      <para>
        If you need to change and update a control in the interrupt
      routine, you can call <function>snd_ctl_notify()</function>. For
      example, 

        <informalexample>
          <programlisting>
<![CDATA[
  snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_VALUE, id_pointer);
]]>
          </programlisting>
        </informalexample>

        This function takes the card pointer, the event-mask, and the
      control id pointer for the notification. The event-mask
      specifies the types of notification, for example, in the above
      example, the change of control values is notified.
      The id pointer is the pointer of struct <structname>snd_ctl_elem_id</structname>
      to be notified.
      You can find some examples in <filename>es1938.c</filename> or
      <filename>es1968.c</filename> for hardware volume interrupts. 
      </para>
    </section>

    <section id="control-interface-tlv">
      <title>Metadata</title>
      <para>
      To provide information about the dB values of a mixer control, use
      on of the <constant>DECLARE_TLV_xxx</constant> macros from
      <filename>&lt;sound/tlv.h&gt;</filename> to define a variable
      containing this information, set the<structfield>tlv.p
      </structfield> field to point to this variable, and include the
      <constant>SNDRV_CTL_ELEM_ACCESS_TLV_READ</constant> flag in the
      <structfield>access</structfield> field; like this:
      <informalexample>
        <programlisting>
<![CDATA[
  static DECLARE_TLV_DB_SCALE(db_scale_my_control, -4050, 150, 0);

  static struct snd_kcontrol_new my_control __devinitdata = {
          ...
          .access = SNDRV_CTL_ELEM_ACCESS_READWRITE |
                    SNDRV_CTL_ELEM_ACCESS_TLV_READ,
          ...
          .tlv.p = db_scale_my_control,
  };
]]>
        </programlisting>
      </informalexample>
      </para>

      <para>
      The <function>DECLARE_TLV_DB_SCALE</function> macro defines
      information about a mixer control where each step in the control's
      value changes the dB value by a constant dB amount.
      The first parameter is the name of the variable to be defined.
      The second parameter is the minimum value, in units of 0.01 dB.
      The third parameter is the step size, in units of 0.01 dB.
      Set the fourth parameter to 1 if the minimum value actually mutes
      the control.
      </para>

      <para>
      The <function>DECLARE_TLV_DB_LINEAR</function> macro defines
      information about a mixer control where the control's value affects
      the output linearly.
      The first parameter is the name of the variable to be defined.
      The second parameter is the minimum value, in units of 0.01 dB.
      The third parameter is the maximum value, in units of 0.01 dB.
      If the minimum value mutes the control, set the second parameter to
      <constant>TLV_DB_GAIN_MUTE</constant>.
      </para>
    </section>

  </chapter>


<!-- ****************************************************** -->
<!-- API for AC97 Codec  -->
<!-- ****************************************************** -->
  <chapter id="api-ac97">
    <title>API for AC97 Codec</title>

    <section>
      <title>General</title>
      <para>
        The ALSA AC97 codec layer is a well-defined one, and you don't
      have to write much code to control it. Only low-level control
      routines are necessary. The AC97 codec API is defined in
      <filename>&lt;sound/ac97_codec.h&gt;</filename>. 
      </para>
    </section>

    <section id="api-ac97-example">
      <title>Full Code Example</title>
      <para>
          <example>
	    <title>Example of AC97 Interface</title>
            <programlisting>
<![CDATA[
  struct mychip {
          ....
          struct snd_ac97 *ac97;
          ....
  };

  static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
                                             unsigned short reg)
  {
          struct mychip *chip = ac97->private_data;
          ....
          /* read a register value here from the codec */
          return the_register_value;
  }

  static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
                                   unsigned short reg, unsigned short val)
  {
          struct mychip *chip = ac97->private_data;
          ....
          /* write the given register value to the codec */
  }

  static int snd_mychip_ac97(struct mychip *chip)
  {
          struct snd_ac97_bus *bus;
          struct snd_ac97_template ac97;
          int err;
          static struct snd_ac97_bus_ops ops = {
                  .write = snd_mychip_ac97_write,
                  .read = snd_mychip_ac97_read,
          };

          err = snd_ac97_bus(chip->card, 0, &ops, NULL, &bus);
          if (err < 0)
                  return err;
          memset(&ac97, 0, sizeof(ac97));
          ac97.private_data = chip;
          return snd_ac97_mixer(bus, &ac97, &chip->ac97);
  }

]]>
          </programlisting>
        </example>
      </para>
    </section>

    <section id="api-ac97-constructor">
      <title>Constructor</title>
      <para>
        To create an ac97 instance, first call <function>snd_ac97_bus</function>
      with an <type>ac97_bus_ops_t</type> record with callback functions.

        <informalexample>
          <programlisting>
<![CDATA[
  struct snd_ac97_bus *bus;
  static struct snd_ac97_bus_ops ops = {
        .write = snd_mychip_ac97_write,
        .read = snd_mychip_ac97_read,
  };

  snd_ac97_bus(card, 0, &ops, NULL, &pbus);
]]>
          </programlisting>
        </informalexample>

      The bus record is shared among all belonging ac97 instances.
      </para>

      <para>
      And then call <function>snd_ac97_mixer()</function> with an
      struct <structname>snd_ac97_template</structname>
      record together with the bus pointer created above.

        <informalexample>
          <programlisting>
<![CDATA[
  struct snd_ac97_template ac97;
  int err;

  memset(&ac97, 0, sizeof(ac97));
  ac97.private_data = chip;
  snd_ac97_mixer(bus, &ac97, &chip->ac97);
]]>
          </programlisting>
        </informalexample>

        where chip-&gt;ac97 is a pointer to a newly created
        <type>ac97_t</type> instance.
        In this case, the chip pointer is set as the private data, so that
        the read/write callback functions can refer to this chip instance.
        This instance is not necessarily stored in the chip
	record.  If you need to change the register values from the
        driver, or need the suspend/resume of ac97 codecs, keep this
        pointer to pass to the corresponding functions.
      </para>
    </section>

    <section id="api-ac97-callbacks">
      <title>Callbacks</title>
      <para>
        The standard callbacks are <structfield>read</structfield> and
      <structfield>write</structfield>. Obviously they 
      correspond to the functions for read and write accesses to the
      hardware low-level codes. 
      </para>

      <para>
        The <structfield>read</structfield> callback returns the
        register value specified in the argument. 

        <informalexample>
          <programlisting>
<![CDATA[
  static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
                                             unsigned short reg)
  {
          struct mychip *chip = ac97->private_data;
          ....
          return the_register_value;
  }
]]>
          </programlisting>
        </informalexample>

        Here, the chip can be cast from ac97-&gt;private_data.
      </para>

      <para>
        Meanwhile, the <structfield>write</structfield> callback is
        used to set the register value. 

        <informalexample>
          <programlisting>
<![CDATA[
  static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
                       unsigned short reg, unsigned short val)
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
      These callbacks are non-atomic like the control API callbacks.
      </para>

      <para>
        There are also other callbacks:
      <structfield>reset</structfield>,
      <structfield>wait</structfield> and
      <structfield>init</structfield>. 
      </para>

      <para>
        The <structfield>reset</structfield> callback is used to reset
      the codec. If the chip requires a special kind of reset, you can
      define this callback. 
      </para>

      <para>
        The <structfield>wait</structfield> callback is used to
      add some waiting time in the standard initialization of the codec. If the
      chip requires the extra waiting time, define this callback. 
      </para>

      <para>
        The <structfield>init</structfield> callback is used for
      additional initialization of the codec.
      </para>
    </section>

    <section id="api-ac97-updating-registers">
      <title>Updating Registers in The Driver</title>
      <para>
        If you need to access to the codec from the driver, you can
      call the following functions:
      <function>snd_ac97_write()</function>,
      <function>snd_ac97_read()</function>,
      <function>snd_ac97_update()</function> and
      <function>snd_ac97_update_bits()</function>. 
      </para>

      <para>
        Both <function>snd_ac97_write()</function> and
        <function>snd_ac97_update()</function> functions are used to
        set a value to the given register
        (<constant>AC97_XXX</constant>). The difference between them is
        that <function>snd_ac97_update()</function> doesn't write a
        value if the given value has been already set, while
        <function>snd_ac97_write()</function> always rewrites the
        value. 

        <informalexample>
          <programlisting>
<![CDATA[
  snd_ac97_write(ac97, AC97_MASTER, 0x8080);
  snd_ac97_update(ac97, AC97_MASTER, 0x8080);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        <function>snd_ac97_read()</function> is used to read the value
        of the given register. For example, 

        <informalexample>
          <programlisting>
<![CDATA[
  value = snd_ac97_read(ac97, AC97_MASTER);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        <function>snd_ac97_update_bits()</function> is used to update
        some bits in the given register.  

        <informalexample>
          <programlisting>
<![CDATA[
  snd_ac97_update_bits(ac97, reg, mask, value);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        Also, there is a function to change the sample rate (of a
        given register such as
        <constant>AC97_PCM_FRONT_DAC_RATE</constant>) when VRA or
        DRA is supported by the codec:
        <function>snd_ac97_set_rate()</function>. 

        <informalexample>
          <programlisting>
<![CDATA[
  snd_ac97_set_rate(ac97, AC97_PCM_FRONT_DAC_RATE, 44100);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        The following registers are available to set the rate:
      <constant>AC97_PCM_MIC_ADC_RATE</constant>,
      <constant>AC97_PCM_FRONT_DAC_RATE</constant>,
      <constant>AC97_PCM_LR_ADC_RATE</constant>,
      <constant>AC97_SPDIF</constant>. When
      <constant>AC97_SPDIF</constant> is specified, the register is
      not really changed but the corresponding IEC958 status bits will
      be updated. 
      </para>
    </section>

    <section id="api-ac97-clock-adjustment">
      <title>Clock Adjustment</title>
      <para>
        In some chips, the clock of the codec isn't 48000 but using a
      PCI clock (to save a quartz!). In this case, change the field
      bus-&gt;clock to the corresponding
      value. For example, intel8x0 
      and es1968 drivers have their own function to read from the clock.
      </para>
    </section>

    <section id="api-ac97-proc-files">
      <title>Proc Files</title>
      <para>
        The ALSA AC97 interface will create a proc file such as
      <filename>/proc/asound/card0/codec97#0/ac97#0-0</filename> and
      <filename>ac97#0-0+regs</filename>. You can refer to these files to
      see the current status and registers of the codec. 
      </para>
    </section>

    <section id="api-ac97-multiple-codecs">
      <title>Multiple Codecs</title>
      <para>
        When there are several codecs on the same card, you need to
      call <function>snd_ac97_mixer()</function> multiple times with
      ac97.num=1 or greater. The <structfield>num</structfield> field
      specifies the codec number. 
      </para>

      <para>
        If you set up multiple codecs, you either need to write
      different callbacks for each codec or check
      ac97-&gt;num in the callback routines. 
      </para>
    </section>

  </chapter>


<!-- ****************************************************** -->
<!-- MIDI (MPU401-UART) Interface  -->
<!-- ****************************************************** -->
  <chapter id="midi-interface">
    <title>MIDI (MPU401-UART) Interface</title>

    <section id="midi-interface-general">
      <title>General</title>
      <para>
        Many soundcards have built-in MIDI (MPU401-UART)
      interfaces. When the soundcard supports the standard MPU401-UART
      interface, most likely you can use the ALSA MPU401-UART API. The
      MPU401-UART API is defined in
      <filename>&lt;sound/mpu401.h&gt;</filename>. 
      </para>

      <para>
        Some soundchips have a similar but slightly different
      implementation of mpu401 stuff. For example, emu10k1 has its own
      mpu401 routines. 
      </para>
    </section>

    <section id="midi-interface-constructor">
      <title>Constructor</title>
      <para>
        To create a rawmidi object, call
      <function>snd_mpu401_uart_new()</function>. 

        <informalexample>
          <programlisting>
<![CDATA[
  struct snd_rawmidi *rmidi;
  snd_mpu401_uart_new(card, 0, MPU401_HW_MPU401, port, info_flags,
                      irq, irq_flags, &rmidi);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        The first argument is the card pointer, and the second is the
      index of this component. You can create up to 8 rawmidi
      devices. 
      </para>

      <para>
        The third argument is the type of the hardware,
      <constant>MPU401_HW_XXX</constant>. If it's not a special one,
      you can use <constant>MPU401_HW_MPU401</constant>. 
      </para>

      <para>
        The 4th argument is the I/O port address. Many
      backward-compatible MPU401 have an I/O port such as 0x330. Or, it
      might be a part of its own PCI I/O region. It depends on the
      chip design. 
      </para>

      <para>
	The 5th argument is a bitflag for additional information.
        When the I/O port address above is part of the PCI I/O
      region, the MPU401 I/O port might have been already allocated
      (reserved) by the driver itself. In such a case, pass a bit flag
      <constant>MPU401_INFO_INTEGRATED</constant>,
      and the mpu401-uart layer will allocate the I/O ports by itself. 
      </para>

	<para>
	When the controller supports only the input or output MIDI stream,
	pass the <constant>MPU401_INFO_INPUT</constant> or
	<constant>MPU401_INFO_OUTPUT</constant> bitflag, respectively.
	Then the rawmidi instance is created as a single stream.
	</para>

	<para>
	<constant>MPU401_INFO_MMIO</constant> bitflag is used to change
	the access method to MMIO (via readb and writeb) instead of
	iob and outb. In this case, you have to pass the iomapped address
	to <function>snd_mpu401_uart_new()</function>.
	</para>

	<para>
	When <constant>MPU401_INFO_TX_IRQ</constant> is set, the output
	stream isn't checked in the default interrupt handler.  The driver
	needs to call <function>snd_mpu401_uart_interrupt_tx()</function>
	by itself to start processing the output stream in the irq handler.
	</para>

      <para>
        Usually, the port address corresponds to the command port and
        port + 1 corresponds to the data port. If not, you may change
        the <structfield>cport</structfield> field of
        struct <structname>snd_mpu401</structname> manually 
        afterward. However, <structname>snd_mpu401</structname> pointer is not
        returned explicitly by
        <function>snd_mpu401_uart_new()</function>. You need to cast
        rmidi-&gt;private_data to
        <structname>snd_mpu401</structname> explicitly, 

        <informalexample>
          <programlisting>
<![CDATA[
  struct snd_mpu401 *mpu;
  mpu = rmidi->private_data;
]]>
          </programlisting>
        </informalexample>

        and reset the cport as you like:

        <informalexample>
          <programlisting>
<![CDATA[
  mpu->cport = my_own_control_port;
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        The 6th argument specifies the irq number for UART. If the irq
      is already allocated, pass 0 to the 7th argument
      (<parameter>irq_flags</parameter>). Otherwise, pass the flags
      for irq allocation 
      (<constant>SA_XXX</constant> bits) to it, and the irq will be
      reserved by the mpu401-uart layer. If the card doesn't generate
      UART interrupts, pass -1 as the irq number. Then a timer
      interrupt will be invoked for polling. 
      </para>
    </section>

    <section id="midi-interface-interrupt-handler">
      <title>Interrupt Handler</title>
      <para>
        When the interrupt is allocated in
      <function>snd_mpu401_uart_new()</function>, the private
      interrupt handler is used, hence you don't have anything else to do
      than creating the mpu401 stuff. Otherwise, you have to call
      <function>snd_mpu401_uart_interrupt()</function> explicitly when
      a UART interrupt is invoked and checked in your own interrupt
      handler.  
      </para>

      <para>
        In this case, you need to pass the private_data of the
        returned rawmidi object from
        <function>snd_mpu401_uart_new()</function> as the second
        argument of <function>snd_mpu401_uart_interrupt()</function>. 

        <informalexample>
          <programlisting>
<![CDATA[
  snd_mpu401_uart_interrupt(irq, rmidi->private_data, regs);
]]>
          </programlisting>
        </informalexample>
      </para>
    </section>

  </chapter>


<!-- ****************************************************** -->
<!-- RawMIDI Interface  -->
<!-- ****************************************************** -->
  <chapter id="rawmidi-interface">
    <title>RawMIDI Interface</title>

    <section id="rawmidi-interface-overview">
      <title>Overview</title>

      <para>
      The raw MIDI interface is used for hardware MIDI ports that can
      be accessed as a byte stream.  It is not used for synthesizer
      chips that do not directly understand MIDI.
      </para>

      <para>
      ALSA handles file and buffer management.  All you have to do is
      to write some code to move data between the buffer and the
      hardware.
      </para>

      <para>
      The rawmidi API is defined in
      <filename>&lt;sound/rawmidi.h&gt;</filename>.
      </para>
    </section>

    <section id="rawmidi-interface-constructor">
      <title>Constructor</title>

      <para>
      To create a rawmidi device, call the
      <function>snd_rawmidi_new</function> function:
        <informalexample>
          <programlisting>
<![CDATA[
  struct snd_rawmidi *rmidi;
  err = snd_rawmidi_new(chip->card, "MyMIDI", 0, outs, ins, &rmidi);
  if (err < 0)
          return err;
  rmidi->private_data = chip;
  strcpy(rmidi->name, "My MIDI");
  rmidi->info_flags = SNDRV_RAWMIDI_INFO_OUTPUT |
                      SNDRV_RAWMIDI_INFO_INPUT |
                      SNDRV_RAWMIDI_INFO_DUPLEX;
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
      The first argument is the card pointer, the second argument is
      the ID string.
      </para>

      <para>
      The third argument is the index of this component.  You can
      create up to 8 rawmidi devices.
      </para>

      <para>
      The fourth and fifth arguments are the number of output and
      input substreams, respectively, of this device (a substream is
      the equivalent of a MIDI port).
      </para>

      <para>
      Set the <structfield>info_flags</structfield> field to specify
      the capabilities of the device.
      Set <constant>SNDRV_RAWMIDI_INFO_OUTPUT</constant> if there is
      at least one output port,
      <constant>SNDRV_RAWMIDI_INFO_INPUT</constant> if there is at
      least one input port,
      and <constant>SNDRV_RAWMIDI_INFO_DUPLEX</constant> if the device
      can handle output and input at the same time.
      </para>

      <para>
      After the rawmidi device is created, you need to set the
      operators (callbacks) for each substream.  There are helper
      functions to set the operators for all the substreams of a device:
        <informalexample>
          <programlisting>
<![CDATA[
  snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_OUTPUT, &snd_mymidi_output_ops);
  snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_INPUT, &snd_mymidi_input_ops);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
      The operators are usually defined like this:
        <informalexample>
          <programlisting>
<![CDATA[
  static struct snd_rawmidi_ops snd_mymidi_output_ops = {
          .open =    snd_mymidi_output_open,
          .close =   snd_mymidi_output_close,
          .trigger = snd_mymidi_output_trigger,
  };
]]>
          </programlisting>
        </informalexample>
      These callbacks are explained in the <link
      linkend="rawmidi-interface-callbacks"><citetitle>Callbacks</citetitle></link>
      section.
      </para>

      <para>
      If there are more than one substream, you should give a
      unique name to each of them:
        <informalexample>
          <programlisting>
<![CDATA[
  struct snd_rawmidi_substream *substream;
  list_for_each_entry(substream,
                      &rmidi->streams[SNDRV_RAWMIDI_STREAM_OUTPUT].substreams,
                      list {
          sprintf(substream->name, "My MIDI Port %d", substream->number + 1);
  }
  /* same for SNDRV_RAWMIDI_STREAM_INPUT */
]]>
          </programlisting>
        </informalexample>
      </para>
    </section>

    <section id="rawmidi-interface-callbacks">
      <title>Callbacks</title>

      <para>
      In all the callbacks, the private data that you've set for the
      rawmidi device can be accessed as
      substream-&gt;rmidi-&gt;private_data.
      <!-- <code> isn't available before DocBook 4.3 -->
      </para>

      <para>
      If there is more than one port, your callbacks can determine the
      port index from the struct snd_rawmidi_substream data passed to each
      callback:
        <informalexample>
          <programlisting>
<![CDATA[
  struct snd_rawmidi_substream *substream;
  int index = substream->number;
]]>
          </programlisting>
        </informalexample>
      </para>

      <section id="rawmidi-interface-op-open">
      <title><function>open</function> callback</title>

        <informalexample>
          <programlisting>
<![CDATA[
  static int snd_xxx_open(struct snd_rawmidi_substream *substream);
]]>
          </programlisting>
        </informalexample>

        <para>
        This is called when a substream is opened.
        You can initialize the hardware here, but you shouldn't
        start transmitting/receiving data yet.
        </para>
      </section>

      <section id="rawmidi-interface-op-close">
      <title><function>close</function> callback</title>

        <informalexample>
          <programlisting>
<![CDATA[
  static int snd_xxx_close(struct snd_rawmidi_substream *substream);
]]>
          </programlisting>
        </informalexample>

        <para>
        Guess what.
        </para>

        <para>
        The <function>open</function> and <function>close</function>
        callbacks of a rawmidi device are serialized with a mutex,
        and can sleep.
        </para>
      </section>

      <section id="rawmidi-interface-op-trigger-out">
      <title><function>trigger</function> callback for output
      substreams</title>

        <informalexample>
          <programlisting>
<![CDATA[
  static void snd_xxx_output_trigger(struct snd_rawmidi_substream *substream, int up);
]]>
          </programlisting>
        </informalexample>

        <para>
        This is called with a nonzero <parameter>up</parameter>
        parameter when there is some data in the substream buffer that
        must be transmitted.
        </para>

        <para>
        To read data from the buffer, call
        <function>snd_rawmidi_transmit_peek</function>.  It will
        return the number of bytes that have been read; this will be
        less than the number of bytes requested when there are no more
        data in the buffer.
        After the data have been transmitted successfully, call
        <function>snd_rawmidi_transmit_ack</function> to remove the
        data from the substream buffer:
          <informalexample>
            <programlisting>
<![CDATA[
  unsigned char data;
  while (snd_rawmidi_transmit_peek(substream, &data, 1) == 1) {
          if (snd_mychip_try_to_transmit(data))
                  snd_rawmidi_transmit_ack(substream, 1);
          else
                  break; /* hardware FIFO full */
  }
]]>
            </programlisting>
          </informalexample>
        </para>

        <para>
        If you know beforehand that the hardware will accept data, you
        can use the <function>snd_rawmidi_transmit</function> function
        which reads some data and removes them from the buffer at once:
          <informalexample>
            <programlisting>
<![CDATA[
  while (snd_mychip_transmit_possible()) {
          unsigned char data;
          if (snd_rawmidi_transmit(substream, &data, 1) != 1)
                  break; /* no more data */
          snd_mychip_transmit(data);
  }
]]>
            </programlisting>
          </informalexample>
        </para>

        <para>
        If you know beforehand how many bytes you can accept, you can
        use a buffer size greater than one with the
        <function>snd_rawmidi_transmit*</function> functions.
        </para>

        <para>
        The <function>trigger</function> callback must not sleep.  If
        the hardware FIFO is full before the substream buffer has been
        emptied, you have to continue transmitting data later, either
        in an interrupt handler, or with a timer if the hardware
        doesn't have a MIDI transmit interrupt.
        </para>

        <para>
        The <function>trigger</function> callback is called with a
        zero <parameter>up</parameter> parameter when the transmission
        of data should be aborted.
        </para>
      </section>

      <section id="rawmidi-interface-op-trigger-in">
      <title><function>trigger</function> callback for input
      substreams</title>

        <informalexample>
          <programlisting>
<![CDATA[
  static void snd_xxx_input_trigger(struct snd_rawmidi_substream *substream, int up);
]]>
          </programlisting>
        </informalexample>

        <para>
        This is called with a nonzero <parameter>up</parameter>
        parameter to enable receiving data, or with a zero
        <parameter>up</parameter> parameter do disable receiving data.
        </para>

        <para>
        The <function>trigger</function> callback must not sleep; the
        actual reading of data from the device is usually done in an
        interrupt handler.
        </para>

        <para>
        When data reception is enabled, your interrupt handler should
        call <function>snd_rawmidi_receive</function> for all received
        data:
          <informalexample>
            <programlisting>
<![CDATA[
  void snd_mychip_midi_interrupt(...)
  {
          while (mychip_midi_available()) {
                  unsigned char data;
                  data = mychip_midi_read();
                  snd_rawmidi_receive(substream, &data, 1);
          }
  }
]]>
            </programlisting>
          </informalexample>
        </para>
      </section>

      <section id="rawmidi-interface-op-drain">
      <title><function>drain</function> callback</title>

        <informalexample>
          <programlisting>
<![CDATA[
  static void snd_xxx_drain(struct snd_rawmidi_substream *substream);
]]>
          </programlisting>
        </informalexample>

        <para>
        This is only used with output substreams.  This function should wait
        until all data read from the substream buffer have been transmitted.
        This ensures that the device can be closed and the driver unloaded
        without losing data.
        </para>

        <para>
        This callback is optional. If you do not set
        <structfield>drain</structfield> in the struct snd_rawmidi_ops
        structure, ALSA will simply wait for 50&nbsp;milliseconds
        instead.
        </para>
      </section>
    </section>

  </chapter>


<!-- ****************************************************** -->
<!-- Miscellaneous Devices  -->
<!-- ****************************************************** -->
  <chapter id="misc-devices">
    <title>Miscellaneous Devices</title>

    <section id="misc-devices-opl3">
      <title>FM OPL3</title>
      <para>
        The FM OPL3 is still used in many chips (mainly for backward
      compatibility). ALSA has a nice OPL3 FM control layer, too. The
      OPL3 API is defined in
      <filename>&lt;sound/opl3.h&gt;</filename>. 
      </para>

      <para>
        FM registers can be directly accessed through the direct-FM API,
      defined in <filename>&lt;sound/asound_fm.h&gt;</filename>. In
      ALSA native mode, FM registers are accessed through
      the Hardware-Dependant Device direct-FM extension API, whereas in
      OSS compatible mode, FM registers can be accessed with the OSS
      direct-FM compatible API in <filename>/dev/dmfmX</filename> device. 
      </para>

      <para>
        To create the OPL3 component, you have two functions to
        call. The first one is a constructor for the <type>opl3_t</type>
        instance. 

        <informalexample>
          <programlisting>
<![CDATA[
  struct snd_opl3 *opl3;
  snd_opl3_create(card, lport, rport, OPL3_HW_OPL3_XXX,
                  integrated, &opl3);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        The first argument is the card pointer, the second one is the
      left port address, and the third is the right port address. In
      most cases, the right port is placed at the left port + 2. 
      </para>

      <para>
        The fourth argument is the hardware type.
      </para>

      <para>
        When the left and right ports have been already allocated by
      the card driver, pass non-zero to the fifth argument
      (<parameter>integrated</parameter>). Otherwise, the opl3 module will
      allocate the specified ports by itself. 
      </para>

      <para>
        When the accessing the hardware requires special method
        instead of the standard I/O access, you can create opl3 instance
        separately with <function>snd_opl3_new()</function>.

        <informalexample>
          <programlisting>
<![CDATA[
  struct snd_opl3 *opl3;
  snd_opl3_new(card, OPL3_HW_OPL3_XXX, &opl3);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
	Then set <structfield>command</structfield>,
	<structfield>private_data</structfield> and
	<structfield>private_free</structfield> for the private
	access function, the private data and the destructor.
	The l_port and r_port are not necessarily set.  Only the
	command must be set properly.  You can retrieve the data
	from the opl3-&gt;private_data field.
      </para>

      <para>
	After creating the opl3 instance via <function>snd_opl3_new()</function>,
	call <function>snd_opl3_init()</function> to initialize the chip to the
	proper state. Note that <function>snd_opl3_create()</function> always
	calls it internally.
      </para>

      <para>
        If the opl3 instance is created successfully, then create a
        hwdep device for this opl3. 

        <informalexample>
          <programlisting>
<![CDATA[
  struct snd_hwdep *opl3hwdep;
  snd_opl3_hwdep_new(opl3, 0, 1, &opl3hwdep);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        The first argument is the <type>opl3_t</type> instance you
      created, and the second is the index number, usually 0. 
      </para>

      <para>
        The third argument is the index-offset for the sequencer
      client assigned to the OPL3 port. When there is an MPU401-UART,
      give 1 for here (UART always takes 0). 
      </para>
    </section>

    <section id="misc-devices-hardware-dependent">
      <title>Hardware-Dependent Devices</title>
      <para>
        Some chips need user-space access for special
      controls or for loading the micro code. In such a case, you can
      create a hwdep (hardware-dependent) device. The hwdep API is
      defined in <filename>&lt;sound/hwdep.h&gt;</filename>. You can
      find examples in opl3 driver or
      <filename>isa/sb/sb16_csp.c</filename>. 
      </para>

      <para>
        The creation of the <type>hwdep</type> instance is done via
        <function>snd_hwdep_new()</function>. 

        <informalexample>
          <programlisting>
<![CDATA[
  struct snd_hwdep *hw;
  snd_hwdep_new(card, "My HWDEP", 0, &hw);
]]>
          </programlisting>
        </informalexample>

        where the third argument is the index number.
      </para>

      <para>
        You can then pass any pointer value to the
        <parameter>private_data</parameter>.
        If you assign a private data, you should define the
        destructor, too. The destructor function is set in
        the <structfield>private_free</structfield> field.  

        <informalexample>
          <programlisting>
<![CDATA[
  struct mydata *p = kmalloc(sizeof(*p), GFP_KERNEL);
  hw->private_data = p;
  hw->private_free = mydata_free;
]]>
          </programlisting>
        </informalexample>

        and the implementation of the destructor would be:

        <informalexample>
          <programlisting>
<![CDATA[
  static void mydata_free(struct snd_hwdep *hw)
  {
          struct mydata *p = hw->private_data;
          kfree(p);
  }
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        The arbitrary file operations can be defined for this
        instance. The file operators are defined in
        the <parameter>ops</parameter> table. For example, assume that
        this chip needs an ioctl. 

        <informalexample>
          <programlisting>
<![CDATA[
  hw->ops.open = mydata_open;
  hw->ops.ioctl = mydata_ioctl;
  hw->ops.release = mydata_release;
]]>
          </programlisting>
        </informalexample>

        And implement the callback functions as you like.
      </para>
    </section>

    <section id="misc-devices-IEC958">
      <title>IEC958 (S/PDIF)</title>
      <para>
        Usually the controls for IEC958 devices are implemented via
      the control interface. There is a macro to compose a name string for
      IEC958 controls, <function>SNDRV_CTL_NAME_IEC958()</function>
      defined in <filename>&lt;include/asound.h&gt;</filename>.  
      </para>

      <para>
        There are some standard controls for IEC958 status bits. These
      controls use the type <type>SNDRV_CTL_ELEM_TYPE_IEC958</type>,
      and the size of element is fixed as 4 bytes array
      (value.iec958.status[x]). For the <structfield>info</structfield>
      callback, you don't specify 
      the value field for this type (the count field must be set,
      though). 
      </para>

      <para>
        <quote>IEC958 Playback Con Mask</quote> is used to return the
      bit-mask for the IEC958 status bits of consumer mode. Similarly,
      <quote>IEC958 Playback Pro Mask</quote> returns the bitmask for
      professional mode. They are read-only controls, and are defined
      as MIXER controls (iface =
      <constant>SNDRV_CTL_ELEM_IFACE_MIXER</constant>).  
      </para>

      <para>
        Meanwhile, <quote>IEC958 Playback Default</quote> control is
      defined for getting and setting the current default IEC958
      bits. Note that this one is usually defined as a PCM control
      (iface = <constant>SNDRV_CTL_ELEM_IFACE_PCM</constant>),
      although in some places it's defined as a MIXER control. 
      </para>

      <para>
        In addition, you can define the control switches to
      enable/disable or to set the raw bit mode. The implementation
      will depend on the chip, but the control should be named as
      <quote>IEC958 xxx</quote>, preferably using
      the <function>SNDRV_CTL_NAME_IEC958()</function> macro. 
      </para>

      <para>
        You can find several cases, for example,
      <filename>pci/emu10k1</filename>,
      <filename>pci/ice1712</filename>, or
      <filename>pci/cmipci.c</filename>.  
      </para>
    </section>

  </chapter>


<!-- ****************************************************** -->
<!-- Buffer and Memory Management  -->
<!-- ****************************************************** -->
  <chapter id="buffer-and-memory">
    <title>Buffer and Memory Management</title>

    <section id="buffer-and-memory-buffer-types">
      <title>Buffer Types</title>
      <para>
        ALSA provides several different buffer allocation functions
      depending on the bus and the architecture. All these have a
      consistent API. The allocation of physically-contiguous pages is
      done via 
      <function>snd_malloc_xxx_pages()</function> function, where xxx
      is the bus type. 
      </para>

      <para>
        The allocation of pages with fallback is
      <function>snd_malloc_xxx_pages_fallback()</function>. This
      function tries to allocate the specified pages but if the pages
      are not available, it tries to reduce the page sizes until
      enough space is found.
      </para>

      <para>
      The release the pages, call
      <function>snd_free_xxx_pages()</function> function. 
      </para>

      <para>
      Usually, ALSA drivers try to allocate and reserve
       a large contiguous physical space
       at the time the module is loaded for the later use.
       This is called <quote>pre-allocation</quote>.
       As already written, you can call the following function at 
       pcm instance construction time (in the case of PCI bus). 

        <informalexample>
          <programlisting>
<![CDATA[
  snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
                                        snd_dma_pci_data(pci), size, max);
]]>
          </programlisting>
        </informalexample>

        where <parameter>size</parameter> is the byte size to be
      pre-allocated and the <parameter>max</parameter> is the maximum
      size to be changed via the <filename>prealloc</filename> proc file.
      The allocator will try to get an area as large as possible
      within the given size. 
      </para>

      <para>
      The second argument (type) and the third argument (device pointer)
      are dependent on the bus.
      In the case of the ISA bus, pass <function>snd_dma_isa_data()</function>
      as the third argument with <constant>SNDRV_DMA_TYPE_DEV</constant> type.
      For the continuous buffer unrelated to the bus can be pre-allocated
      with <constant>SNDRV_DMA_TYPE_CONTINUOUS</constant> type and the
      <function>snd_dma_continuous_data(GFP_KERNEL)</function> device pointer,
      where <constant>GFP_KERNEL</constant> is the kernel allocation flag to
      use.
      For the PCI scatter-gather buffers, use
      <constant>SNDRV_DMA_TYPE_DEV_SG</constant> with
      <function>snd_dma_pci_data(pci)</function>
      (see the 
          <link linkend="buffer-and-memory-non-contiguous"><citetitle>Non-Contiguous Buffers
          </citetitle></link> section).
      </para>

      <para>
        Once the buffer is pre-allocated, you can use the
        allocator in the <structfield>hw_params</structfield> callback: 

        <informalexample>
          <programlisting>
<![CDATA[
  snd_pcm_lib_malloc_pages(substream, size);
]]>
          </programlisting>
        </informalexample>

        Note that you have to pre-allocate to use this function.
      </para>
    </section>

    <section id="buffer-and-memory-external-hardware">
      <title>External Hardware Buffers</title>
      <para>
        Some chips have their own hardware buffers and the DMA
      transfer from the host memory is not available. In such a case,
      you need to either 1) copy/set the audio data directly to the
      external hardware buffer, or 2) make an intermediate buffer and
      copy/set the data from it to the external hardware buffer in
      interrupts (or in tasklets, preferably).
      </para>

      <para>
        The first case works fine if the external hardware buffer is large
      enough.  This method doesn't need any extra buffers and thus is
      more effective. You need to define the
      <structfield>copy</structfield> and
      <structfield>silence</structfield> callbacks for 
      the data transfer. However, there is a drawback: it cannot
      be mmapped. The examples are GUS's GF1 PCM or emu8000's
      wavetable PCM. 
      </para>

      <para>
        The second case allows for mmap on the buffer, although you have
      to handle an interrupt or a tasklet to transfer the data
      from the intermediate buffer to the hardware buffer. You can find an
      example in the vxpocket driver. 
      </para>

      <para>
        Another case is when the chip uses a PCI memory-map
      region for the buffer instead of the host memory. In this case,
      mmap is available only on certain architectures like the Intel one.
      In non-mmap mode, the data cannot be transferred as in the normal
      way. Thus you need to define the <structfield>copy</structfield> and
      <structfield>silence</structfield> callbacks as well, 
      as in the cases above. The examples are found in
      <filename>rme32.c</filename> and <filename>rme96.c</filename>. 
      </para>

      <para>
        The implementation of the <structfield>copy</structfield> and
        <structfield>silence</structfield> callbacks depends upon 
        whether the hardware supports interleaved or non-interleaved
        samples. The <structfield>copy</structfield> callback is
        defined like below, a bit 
        differently depending whether the direction is playback or
        capture: 

        <informalexample>
          <programlisting>
<![CDATA[
  static int playback_copy(struct snd_pcm_substream *substream, int channel,
               snd_pcm_uframes_t pos, void *src, snd_pcm_uframes_t count);
  static int capture_copy(struct snd_pcm_substream *substream, int channel,
               snd_pcm_uframes_t pos, void *dst, snd_pcm_uframes_t count);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        In the case of interleaved samples, the second argument
      (<parameter>channel</parameter>) is not used. The third argument
      (<parameter>pos</parameter>) points the 
      current position offset in frames. 
      </para>

      <para>
        The meaning of the fourth argument is different between
      playback and capture. For playback, it holds the source data
      pointer, and for capture, it's the destination data pointer. 
      </para>

      <para>
        The last argument is the number of frames to be copied.
      </para>

      <para>
        What you have to do in this callback is again different
        between playback and capture directions. In the
        playback case, you copy the given amount of data
        (<parameter>count</parameter>) at the specified pointer
        (<parameter>src</parameter>) to the specified offset
        (<parameter>pos</parameter>) on the hardware buffer. When
        coded like memcpy-like way, the copy would be like: 

        <informalexample>
          <programlisting>
<![CDATA[
  my_memcpy(my_buffer + frames_to_bytes(runtime, pos), src,
            frames_to_bytes(runtime, count));
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        For the capture direction, you copy the given amount of
        data (<parameter>count</parameter>) at the specified offset
        (<parameter>pos</parameter>) on the hardware buffer to the
        specified pointer (<parameter>dst</parameter>). 

        <informalexample>
          <programlisting>
<![CDATA[
  my_memcpy(dst, my_buffer + frames_to_bytes(runtime, pos),
            frames_to_bytes(runtime, count));
]]>
          </programlisting>
        </informalexample>

        Note that both the position and the amount of data are given
      in frames. 
      </para>

      <para>
        In the case of non-interleaved samples, the implementation
      will be a bit more complicated. 
      </para>

      <para>
        You need to check the channel argument, and if it's -1, copy
      the whole channels. Otherwise, you have to copy only the
      specified channel. Please check
      <filename>isa/gus/gus_pcm.c</filename> as an example. 
      </para>

      <para>
        The <structfield>silence</structfield> callback is also
        implemented in a similar way. 

        <informalexample>
          <programlisting>
<![CDATA[
  static int silence(struct snd_pcm_substream *substream, int channel,
                     snd_pcm_uframes_t pos, snd_pcm_uframes_t count);
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        The meanings of arguments are the same as in the
      <structfield>copy</structfield> 
      callback, although there is no <parameter>src/dst</parameter>
      argument. In the case of interleaved samples, the channel
      argument has no meaning, as well as on
      <structfield>copy</structfield> callback.  
      </para>

      <para>
        The role of <structfield>silence</structfield> callback is to
        set the given amount 
        (<parameter>count</parameter>) of silence data at the
        specified offset (<parameter>pos</parameter>) on the hardware
        buffer. Suppose that the data format is signed (that is, the
        silent-data is 0), and the implementation using a memset-like
        function would be like: 

        <informalexample>
          <programlisting>
<![CDATA[
  my_memcpy(my_buffer + frames_to_bytes(runtime, pos), 0,
            frames_to_bytes(runtime, count));
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        In the case of non-interleaved samples, again, the
      implementation becomes a bit more complicated. See, for example,
      <filename>isa/gus/gus_pcm.c</filename>. 
      </para>
    </section>

    <section id="buffer-and-memory-non-contiguous">
      <title>Non-Contiguous Buffers</title>
      <para>
        If your hardware supports the page table as in emu10k1 or the
      buffer descriptors as in via82xx, you can use the scatter-gather
      (SG) DMA. ALSA provides an interface for handling SG-buffers.
      The API is provided in <filename>&lt;sound/pcm.h&gt;</filename>. 
      </para>

      <para>
        For creating the SG-buffer handler, call
        <function>snd_pcm_lib_preallocate_pages()</function> or
        <function>snd_pcm_lib_preallocate_pages_for_all()</function>
        with <constant>SNDRV_DMA_TYPE_DEV_SG</constant>
	in the PCM constructor like other PCI pre-allocator.
        You need to pass <function>snd_dma_pci_data(pci)</function>,
        where pci is the struct <structname>pci_dev</structname> pointer
        of the chip as well.
        The <type>struct snd_sg_buf</type> instance is created as
        substream-&gt;dma_private. You can cast
        the pointer like: 

        <informalexample>
          <programlisting>
<![CDATA[
  struct snd_sg_buf *sgbuf = (struct snd_sg_buf *)substream->dma_private;
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        Then call <function>snd_pcm_lib_malloc_pages()</function>
      in the <structfield>hw_params</structfield> callback
      as well as in the case of normal PCI buffer.
      The SG-buffer handler will allocate the non-contiguous kernel
      pages of the given size and map them onto the virtually contiguous
      memory.  The virtual pointer is addressed in runtime-&gt;dma_area.
      The physical address (runtime-&gt;dma_addr) is set to zero,
      because the buffer is physically non-contiguous.
      The physical address table is set up in sgbuf-&gt;table.
      You can get the physical address at a certain offset via
      <function>snd_pcm_sgbuf_get_addr()</function>. 
      </para>

      <para>
        When a SG-handler is used, you need to set
      <function>snd_pcm_sgbuf_ops_page</function> as
      the <structfield>page</structfield> callback.
      (See <link linkend="pcm-interface-operators-page-callback">
      <citetitle>page callback section</citetitle></link>.)
      </para>

      <para>
        To release the data, call
      <function>snd_pcm_lib_free_pages()</function> in the
      <structfield>hw_free</structfield> callback as usual.
      </para>
    </section>

    <section id="buffer-and-memory-vmalloced">
      <title>Vmalloc'ed Buffers</title>
      <para>
        It's possible to use a buffer allocated via
      <function>vmalloc</function>, for example, for an intermediate
      buffer. Since the allocated pages are not contiguous, you need
      to set the <structfield>page</structfield> callback to obtain
      the physical address at every offset. 
      </para>

      <para>
        The implementation of <structfield>page</structfield> callback
        would be like this: 

        <informalexample>
          <programlisting>
<![CDATA[
  #include <linux/vmalloc.h>

  /* get the physical page pointer on the given offset */
  static struct page *mychip_page(struct snd_pcm_substream *substream,
                                  unsigned long offset)
  {
          void *pageptr = substream->runtime->dma_area + offset;
          return vmalloc_to_page(pageptr);
  }
]]>
          </programlisting>
        </informalexample>
      </para>
    </section>

  </chapter>


<!-- ****************************************************** -->
<!-- Proc Interface  -->
<!-- ****************************************************** -->
  <chapter id="proc-interface">
    <title>Proc Interface</title>
    <para>
      ALSA provides an easy interface for procfs. The proc files are
      very useful for debugging. I recommend you set up proc files if
      you write a driver and want to get a running status or register
      dumps. The API is found in
      <filename>&lt;sound/info.h&gt;</filename>. 
    </para>

    <para>
      To create a proc file, call
      <function>snd_card_proc_new()</function>. 

      <informalexample>
        <programlisting>
<![CDATA[
  struct snd_info_entry *entry;
  int err = snd_card_proc_new(card, "my-file", &entry);
]]>
        </programlisting>
      </informalexample>

      where the second argument specifies the name of the proc file to be
    created. The above example will create a file
    <filename>my-file</filename> under the card directory,
    e.g. <filename>/proc/asound/card0/my-file</filename>. 
    </para>

    <para>
    Like other components, the proc entry created via
    <function>snd_card_proc_new()</function> will be registered and
    released automatically in the card registration and release
    functions.
    </para>

    <para>
      When the creation is successful, the function stores a new
    instance in the pointer given in the third argument.
    It is initialized as a text proc file for read only.  To use
    this proc file as a read-only text file as it is, set the read
    callback with a private data via 
     <function>snd_info_set_text_ops()</function>.

      <informalexample>
        <programlisting>
<![CDATA[
  snd_info_set_text_ops(entry, chip, my_proc_read);
]]>
        </programlisting>
      </informalexample>
    
    where the second argument (<parameter>chip</parameter>) is the
    private data to be used in the callbacks. The third parameter
    specifies the read buffer size and the fourth
    (<parameter>my_proc_read</parameter>) is the callback function, which
    is defined like

      <informalexample>
        <programlisting>
<![CDATA[
  static void my_proc_read(struct snd_info_entry *entry,
                           struct snd_info_buffer *buffer);
]]>
        </programlisting>
      </informalexample>
    
    </para>

    <para>
    In the read callback, use <function>snd_iprintf()</function> for
    output strings, which works just like normal
    <function>printf()</function>.  For example,

      <informalexample>
        <programlisting>
<![CDATA[
  static void my_proc_read(struct snd_info_entry *entry,
                           struct snd_info_buffer *buffer)
  {
          struct my_chip *chip = entry->private_data;

          snd_iprintf(buffer, "This is my chip!\n");
          snd_iprintf(buffer, "Port = %ld\n", chip->port);
  }
]]>
        </programlisting>
      </informalexample>
    </para>

    <para>
    The file permissions can be changed afterwards.  As default, it's
    set as read only for all users.  If you want to add write
    permission for the user (root as default), do as follows:

      <informalexample>
        <programlisting>
<![CDATA[
 entry->mode = S_IFREG | S_IRUGO | S_IWUSR;
]]>
        </programlisting>
      </informalexample>

    and set the write buffer size and the callback

      <informalexample>
        <programlisting>
<![CDATA[
  entry->c.text.write = my_proc_write;
]]>
        </programlisting>
      </informalexample>
    </para>

    <para>
      For the write callback, you can use
    <function>snd_info_get_line()</function> to get a text line, and
    <function>snd_info_get_str()</function> to retrieve a string from
    the line. Some examples are found in
    <filename>core/oss/mixer_oss.c</filename>, core/oss/and
    <filename>pcm_oss.c</filename>. 
    </para>

    <para>
      For a raw-data proc-file, set the attributes as follows:

      <informalexample>
        <programlisting>
<![CDATA[
  static struct snd_info_entry_ops my_file_io_ops = {
          .read = my_file_io_read,
  };

  entry->content = SNDRV_INFO_CONTENT_DATA;
  entry->private_data = chip;
  entry->c.ops = &my_file_io_ops;
  entry->size = 4096;
  entry->mode = S_IFREG | S_IRUGO;
]]>
        </programlisting>
      </informalexample>
    </para>

    <para>
      The callback is much more complicated than the text-file
      version. You need to use a low-level I/O functions such as
      <function>copy_from/to_user()</function> to transfer the
      data.

      <informalexample>
        <programlisting>
<![CDATA[
  static long my_file_io_read(struct snd_info_entry *entry,
                              void *file_private_data,
                              struct file *file,
                              char *buf,
                              unsigned long count,
                              unsigned long pos)
  {
          long size = count;
          if (pos + size > local_max_size)
                  size = local_max_size - pos;
          if (copy_to_user(buf, local_data + pos, size))
                  return -EFAULT;
          return size;
  }
]]>
        </programlisting>
      </informalexample>
    </para>

  </chapter>


<!-- ****************************************************** -->
<!-- Power Management  -->
<!-- ****************************************************** -->
  <chapter id="power-management">
    <title>Power Management</title>
    <para>
      If the chip is supposed to work with suspend/resume
      functions, you need to add power-management code to the
      driver. The additional code for power-management should be
      <function>ifdef</function>'ed with
      <constant>CONFIG_PM</constant>. 
    </para>

	<para>
	If the driver <emphasis>fully</emphasis> supports suspend/resume
	that is, the device can be
	properly resumed to its state when suspend was called,
	you can set the <constant>SNDRV_PCM_INFO_RESUME</constant> flag
	in the pcm info field.  Usually, this is possible when the
	registers of the chip can be safely saved and restored to
	RAM. If this is set, the trigger callback is called with
	<constant>SNDRV_PCM_TRIGGER_RESUME</constant> after the resume
	callback completes. 
	</para>

	<para>
	Even if the driver doesn't support PM fully but 
	partial suspend/resume is still possible, it's still worthy to
	implement suspend/resume callbacks. In such a case, applications
	would reset the status by calling
	<function>snd_pcm_prepare()</function> and restart the stream
	appropriately.  Hence, you can define suspend/resume callbacks
	below but don't set <constant>SNDRV_PCM_INFO_RESUME</constant>
	info flag to the PCM.
	</para>
	
	<para>
	Note that the trigger with SUSPEND can always be called when
	<function>snd_pcm_suspend_all</function> is called,
	regardless of the <constant>SNDRV_PCM_INFO_RESUME</constant> flag.
	The <constant>RESUME</constant> flag affects only the behavior
	of <function>snd_pcm_resume()</function>.
	(Thus, in theory,
	<constant>SNDRV_PCM_TRIGGER_RESUME</constant> isn't needed
	to be handled in the trigger callback when no
	<constant>SNDRV_PCM_INFO_RESUME</constant> flag is set.  But,
	it's better to keep it for compatibility reasons.)
	</para>
    <para>
      In the earlier version of ALSA drivers, a common
      power-management layer was provided, but it has been removed.
      The driver needs to define the suspend/resume hooks according to
      the bus the device is connected to.  In the case of PCI drivers, the
      callbacks look like below:

      <informalexample>
        <programlisting>
<![CDATA[
  #ifdef CONFIG_PM
  static int snd_my_suspend(struct pci_dev *pci, pm_message_t state)
  {
          .... /* do things for suspend */
          return 0;
  }
  static int snd_my_resume(struct pci_dev *pci)
  {
          .... /* do things for suspend */
          return 0;
  }
  #endif
]]>
        </programlisting>
      </informalexample>
    </para>

    <para>
      The scheme of the real suspend job is as follows.

      <orderedlist>
        <listitem><para>Retrieve the card and the chip data.</para></listitem>
        <listitem><para>Call <function>snd_power_change_state()</function> with
	  <constant>SNDRV_CTL_POWER_D3hot</constant> to change the
	  power status.</para></listitem>
        <listitem><para>Call <function>snd_pcm_suspend_all()</function> to suspend the running PCM streams.</para></listitem>
	<listitem><para>If AC97 codecs are used, call
	<function>snd_ac97_suspend()</function> for each codec.</para></listitem>
        <listitem><para>Save the register values if necessary.</para></listitem>
        <listitem><para>Stop the hardware if necessary.</para></listitem>
        <listitem><para>Disable the PCI device by calling
	  <function>pci_disable_device()</function>.  Then, call
          <function>pci_save_state()</function> at last.</para></listitem>
      </orderedlist>
    </para>

    <para>
      A typical code would be like:

      <informalexample>
        <programlisting>
<![CDATA[
  static int mychip_suspend(struct pci_dev *pci, pm_message_t state)
  {
          /* (1) */
          struct snd_card *card = pci_get_drvdata(pci);
          struct mychip *chip = card->private_data;
          /* (2) */
          snd_power_change_state(card, SNDRV_CTL_POWER_D3hot);
          /* (3) */
          snd_pcm_suspend_all(chip->pcm);
          /* (4) */
          snd_ac97_suspend(chip->ac97);
          /* (5) */
          snd_mychip_save_registers(chip);
          /* (6) */
          snd_mychip_stop_hardware(chip);
          /* (7) */
          pci_disable_device(pci);
          pci_save_state(pci);
          return 0;
  }
]]>
        </programlisting>
      </informalexample>
    </para>

    <para>
    The scheme of the real resume job is as follows.

    <orderedlist>
    <listitem><para>Retrieve the card and the chip data.</para></listitem>
    <listitem><para>Set up PCI. First, call <function>pci_restore_state()</function>.
    	Then enable the pci device again by calling <function>pci_enable_device()</function>.
	Call <function>pci_set_master()</function> if necessary, too.</para></listitem>
    <listitem><para>Re-initialize the chip.</para></listitem>
    <listitem><para>Restore the saved registers if necessary.</para></listitem>
    <listitem><para>Resume the mixer, e.g. calling
    <function>snd_ac97_resume()</function>.</para></listitem>
    <listitem><para>Restart the hardware (if any).</para></listitem>
    <listitem><para>Call <function>snd_power_change_state()</function> with
	<constant>SNDRV_CTL_POWER_D0</constant> to notify the processes.</para></listitem>
    </orderedlist>
    </para>

    <para>
    A typical code would be like:

      <informalexample>
        <programlisting>
<![CDATA[
  static int mychip_resume(struct pci_dev *pci)
  {
          /* (1) */
          struct snd_card *card = pci_get_drvdata(pci);
          struct mychip *chip = card->private_data;
          /* (2) */
          pci_restore_state(pci);
          pci_enable_device(pci);
          pci_set_master(pci);
          /* (3) */
          snd_mychip_reinit_chip(chip);
          /* (4) */
          snd_mychip_restore_registers(chip);
          /* (5) */
          snd_ac97_resume(chip->ac97);
          /* (6) */
          snd_mychip_restart_chip(chip);
          /* (7) */
          snd_power_change_state(card, SNDRV_CTL_POWER_D0);
          return 0;
  }
]]>
        </programlisting>
      </informalexample>
    </para>

    <para>
	As shown in the above, it's better to save registers after
	suspending the PCM operations via
	<function>snd_pcm_suspend_all()</function> or
	<function>snd_pcm_suspend()</function>.  It means that the PCM
	streams are already stoppped when the register snapshot is
	taken.  But, remember that you don't have to restart the PCM
	stream in the resume callback. It'll be restarted via 
	trigger call with <constant>SNDRV_PCM_TRIGGER_RESUME</constant>
	when necessary.
    </para>

    <para>
      OK, we have all callbacks now. Let's set them up. In the
      initialization of the card, make sure that you can get the chip
      data from the card instance, typically via
      <structfield>private_data</structfield> field, in case you
      created the chip data individually.

      <informalexample>
        <programlisting>
<![CDATA[
  static int __devinit snd_mychip_probe(struct pci_dev *pci,
                               const struct pci_device_id *pci_id)
  {
          ....
          struct snd_card *card;
          struct mychip *chip;
          int err;
          ....
          err = snd_card_create(index[dev], id[dev], THIS_MODULE, 0, &card);
          ....
          chip = kzalloc(sizeof(*chip), GFP_KERNEL);
          ....
          card->private_data = chip;
          ....
  }
]]>
        </programlisting>
      </informalexample>

	When you created the chip data with
	<function>snd_card_create()</function>, it's anyway accessible
	via <structfield>private_data</structfield> field.

      <informalexample>
        <programlisting>
<![CDATA[
  static int __devinit snd_mychip_probe(struct pci_dev *pci,
                               const struct pci_device_id *pci_id)
  {
          ....
          struct snd_card *card;
          struct mychip *chip;
          int err;
          ....
          err = snd_card_create(index[dev], id[dev], THIS_MODULE,
                                sizeof(struct mychip), &card);
          ....
          chip = card->private_data;
          ....
  }
]]>
        </programlisting>
      </informalexample>

    </para>

    <para>
      If you need a space to save the registers, allocate the
	buffer for it here, too, since it would be fatal
    if you cannot allocate a memory in the suspend phase.
    The allocated buffer should be released in the corresponding
    destructor.
    </para>

    <para>
      And next, set suspend/resume callbacks to the pci_driver.

      <informalexample>
        <programlisting>
<![CDATA[
  static struct pci_driver driver = {
          .name = "My Chip",
          .id_table = snd_my_ids,
          .probe = snd_my_probe,
          .remove = __devexit_p(snd_my_remove),
  #ifdef CONFIG_PM
          .suspend = snd_my_suspend,
          .resume = snd_my_resume,
  #endif
  };
]]>
        </programlisting>
      </informalexample>
    </para>

  </chapter>


<!-- ****************************************************** -->
<!-- Module Parameters  -->
<!-- ****************************************************** -->
  <chapter id="module-parameters">
    <title>Module Parameters</title>
    <para>
      There are standard module options for ALSA. At least, each
      module should have the <parameter>index</parameter>,
      <parameter>id</parameter> and <parameter>enable</parameter>
      options. 
    </para>

    <para>
      If the module supports multiple cards (usually up to
      8 = <constant>SNDRV_CARDS</constant> cards), they should be
      arrays. The default initial values are defined already as
      constants for easier programming:

      <informalexample>
        <programlisting>
<![CDATA[
  static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
  static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
  static int enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
]]>
        </programlisting>
      </informalexample>
    </para>

    <para>
      If the module supports only a single card, they could be single
    variables, instead.  <parameter>enable</parameter> option is not
    always necessary in this case, but it would be better to have a
    dummy option for compatibility.
    </para>

    <para>
      The module parameters must be declared with the standard
    <function>module_param()()</function>,
    <function>module_param_array()()</function> and
    <function>MODULE_PARM_DESC()</function> macros.
    </para>

    <para>
      The typical coding would be like below:

      <informalexample>
        <programlisting>
<![CDATA[
  #define CARD_NAME "My Chip"

  module_param_array(index, int, NULL, 0444);
  MODULE_PARM_DESC(index, "Index value for " CARD_NAME " soundcard.");
  module_param_array(id, charp, NULL, 0444);
  MODULE_PARM_DESC(id, "ID string for " CARD_NAME " soundcard.");
  module_param_array(enable, bool, NULL, 0444);
  MODULE_PARM_DESC(enable, "Enable " CARD_NAME " soundcard.");
]]>
        </programlisting>
      </informalexample>
    </para>

    <para>
      Also, don't forget to define the module description, classes,
      license and devices. Especially, the recent modprobe requires to
      define the module license as GPL, etc., otherwise the system is
      shown as <quote>tainted</quote>. 

      <informalexample>
        <programlisting>
<![CDATA[
  MODULE_DESCRIPTION("My Chip");
  MODULE_LICENSE("GPL");
  MODULE_SUPPORTED_DEVICE("{{Vendor,My Chip Name}}");
]]>
        </programlisting>
      </informalexample>
    </para>

  </chapter>


<!-- ****************************************************** -->
<!-- How To Put Your Driver  -->
<!-- ****************************************************** -->
  <chapter id="how-to-put-your-driver">
    <title>How To Put Your Driver Into ALSA Tree</title>
	<section>
	<title>General</title>
	<para>
	So far, you've learned how to write the driver codes.
	And you might have a question now: how to put my own
	driver into the ALSA driver tree?
	Here (finally :) the standard procedure is described briefly.
	</para>

	<para>
	Suppose that you create a new PCI driver for the card
	<quote>xyz</quote>.  The card module name would be
	snd-xyz.  The new driver is usually put into the alsa-driver
	tree, <filename>alsa-driver/pci</filename> directory in
	the case of PCI cards.
	Then the driver is evaluated, audited and tested
	by developers and users.  After a certain time, the driver
	will go to the alsa-kernel tree (to the corresponding directory,
	such as <filename>alsa-kernel/pci</filename>) and eventually
 	will be integrated into the Linux 2.6 tree (the directory would be
	<filename>linux/sound/pci</filename>).
	</para>

	<para>
	In the following sections, the driver code is supposed
	to be put into alsa-driver tree. The two cases are covered:
	a driver consisting of a single source file and one consisting
	of several source files.
	</para>
	</section>

	<section>
	<title>Driver with A Single Source File</title>
	<para>
	<orderedlist>
	<listitem>
	<para>
	Modify alsa-driver/pci/Makefile
	</para>

	<para>
	Suppose you have a file xyz.c.  Add the following
	two lines
      <informalexample>
        <programlisting>
<![CDATA[
  snd-xyz-objs := xyz.o
  obj-$(CONFIG_SND_XYZ) += snd-xyz.o
]]>
        </programlisting>
      </informalexample>
	</para>
	</listitem>

	<listitem>
	<para>
	Create the Kconfig entry
	</para>

	<para>
	Add the new entry of Kconfig for your xyz driver.
      <informalexample>
        <programlisting>
<![CDATA[
  config SND_XYZ
          tristate "Foobar XYZ"
          depends on SND
          select SND_PCM
          help
            Say Y here to include support for Foobar XYZ soundcard.

            To compile this driver as a module, choose M here: the module
            will be called snd-xyz.
]]>
        </programlisting>
      </informalexample>

	the line, select SND_PCM, specifies that the driver xyz supports
	PCM.  In addition to SND_PCM, the following components are
	supported for select command:
	SND_RAWMIDI, SND_TIMER, SND_HWDEP, SND_MPU401_UART,
	SND_OPL3_LIB, SND_OPL4_LIB, SND_VX_LIB, SND_AC97_CODEC.
	Add the select command for each supported component.
	</para>

	<para>
	Note that some selections imply the lowlevel selections.
	For example, PCM includes TIMER, MPU401_UART includes RAWMIDI,
	AC97_CODEC includes PCM, and OPL3_LIB includes HWDEP.
	You don't need to give the lowlevel selections again.
	</para>

	<para>
	For the details of Kconfig script, refer to the kbuild
	documentation.
	</para>

	</listitem>

	<listitem>
	<para>
	Run cvscompile script to re-generate the configure script and
	build the whole stuff again.
	</para>
	</listitem>
	</orderedlist>
	</para>
	</section>

	<section>
	<title>Drivers with Several Source Files</title>
	<para>
	Suppose that the driver snd-xyz have several source files.
	They are located in the new subdirectory,
	pci/xyz.

	<orderedlist>
	<listitem>
	<para>
	Add a new directory (<filename>xyz</filename>) in
	<filename>alsa-driver/pci/Makefile</filename> as below

      <informalexample>
        <programlisting>
<![CDATA[
  obj-$(CONFIG_SND) += xyz/
]]>
        </programlisting>
      </informalexample>
	</para>
	</listitem>

	<listitem>
	<para>
	Under the directory <filename>xyz</filename>, create a Makefile

      <example>
	<title>Sample Makefile for a driver xyz</title>
        <programlisting>
<![CDATA[
  ifndef SND_TOPDIR
  SND_TOPDIR=../..
  endif

  include $(SND_TOPDIR)/toplevel.config
  include $(SND_TOPDIR)/Makefile.conf

  snd-xyz-objs := xyz.o abc.o def.o

  obj-$(CONFIG_SND_XYZ) += snd-xyz.o

  include $(SND_TOPDIR)/Rules.make
]]>
        </programlisting>
      </example>
	</para>
	</listitem>

	<listitem>
	<para>
	Create the Kconfig entry
	</para>

	<para>
	This procedure is as same as in the last section.
	</para>
	</listitem>

	<listitem>
	<para>
	Run cvscompile script to re-generate the configure script and
	build the whole stuff again.
	</para>
	</listitem>
	</orderedlist>
	</para>
	</section>

  </chapter>

<!-- ****************************************************** -->
<!-- Useful Functions  -->
<!-- ****************************************************** -->
  <chapter id="useful-functions">
    <title>Useful Functions</title>

    <section id="useful-functions-snd-printk">
      <title><function>snd_printk()</function> and friends</title>
      <para>
        ALSA provides a verbose version of the
      <function>printk()</function> function. If a kernel config
      <constant>CONFIG_SND_VERBOSE_PRINTK</constant> is set, this
      function prints the given message together with the file name
      and the line of the caller. The <constant>KERN_XXX</constant>
      prefix is processed as 
      well as the original <function>printk()</function> does, so it's
      recommended to add this prefix, e.g. 

        <informalexample>
          <programlisting>
<![CDATA[
  snd_printk(KERN_ERR "Oh my, sorry, it's extremely bad!\n");
]]>
          </programlisting>
        </informalexample>
      </para>

      <para>
        There are also <function>printk()</function>'s for
      debugging. <function>snd_printd()</function> can be used for
      general debugging purposes. If
      <constant>CONFIG_SND_DEBUG</constant> is set, this function is
      compiled, and works just like
      <function>snd_printk()</function>. If the ALSA is compiled
      without the debugging flag, it's ignored. 
      </para>

      <para>
        <function>snd_printdd()</function> is compiled in only when
      <constant>CONFIG_SND_DEBUG_VERBOSE</constant> is set. Please note
      that <constant>CONFIG_SND_DEBUG_VERBOSE</constant> is not set as default
      even if you configure the alsa-driver with
      <option>--with-debug=full</option> option. You need to give
      explicitly <option>--with-debug=detect</option> option instead. 
      </para>
    </section>

    <section id="useful-functions-snd-bug">
      <title><function>snd_BUG()</function></title>
      <para>
        It shows the <computeroutput>BUG?</computeroutput> message and
      stack trace as well as <function>snd_BUG_ON</function> at the point.
      It's useful to show that a fatal error happens there. 
      </para>
      <para>
	 When no debug flag is set, this macro is ignored. 
      </para>
    </section>

    <section id="useful-functions-snd-bug-on">
      <title><function>snd_BUG_ON()</function></title>
      <para>
        <function>snd_BUG_ON()</function> macro is similar with
	<function>WARN_ON()</function> macro. For example,  

        <informalexample>
          <programlisting>
<![CDATA[
  snd_BUG_ON(!pointer);
]]>
          </programlisting>
        </informalexample>

	or it can be used as the condition,
        <informalexample>
          <programlisting>
<![CDATA[
  if (snd_BUG_ON(non_zero_is_bug))
          return -EINVAL;
]]>
          </programlisting>
        </informalexample>

      </para>

      <para>
        The macro takes an conditional expression to evaluate.
	When <constant>CONFIG_SND_DEBUG</constant>, is set, the
	expression is actually evaluated. If it's non-zero, it shows
	the warning message such as
	<computeroutput>BUG? (xxx)</computeroutput>
	normally followed by stack trace.  It returns the evaluated
	value.
	When no <constant>CONFIG_SND_DEBUG</constant> is set, this
	macro always returns zero.
      </para>

    </section>

  </chapter>


<!-- ****************************************************** -->
<!-- Acknowledgments  -->
<!-- ****************************************************** -->
  <chapter id="acknowledgments">
    <title>Acknowledgments</title>
    <para>
      I would like to thank Phil Kerr for his help for improvement and
      corrections of this document. 
    </para>
    <para>
    Kevin Conder reformatted the original plain-text to the
    DocBook format.
    </para>
    <para>
    Giuliano Pochini corrected typos and contributed the example codes
    in the hardware constraints section.
    </para>
  </chapter>
</book>

Privacy Policy