path: root/Documentation/scsi
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authorDouglas Gilbert <dgilbert@interlog.com>2010-12-23 21:40:37 -0500
committerJames Bottomley <James.Bottomley@suse.de>2011-01-24 12:21:39 -0600
commit75c65a5edaa8b815c1428314eaf696788f1ebbdb (patch)
treebdb28de023e48dcc0f848bf8fb55310a9caa0322 /Documentation/scsi
parent4977c82504f58d7579acabd21688183eaa8768fb (diff)
[SCSI] scsi_mid_low_api.txt recommend resid usage
As discussed in a thread on this list titled: "RFC: short reads on block devices" this patch adds recommendations for LLDs to set resid when there might be uncertainty about how much data has been returned by a device. This patch inline and attached] is against scsi-misc-2.6.git Signed-off-by: Douglas Gilbert <dgilbert@interlog.com> Signed-off-by: James Bottomley <James.Bottomley@suse.de>
Diffstat (limited to 'Documentation/scsi')
1 files changed, 13 insertions, 1 deletions
diff --git a/Documentation/scsi/scsi_mid_low_api.txt b/Documentation/scsi/scsi_mid_low_api.txt
index df322c103466..5f17d29c59b5 100644
--- a/Documentation/scsi/scsi_mid_low_api.txt
+++ b/Documentation/scsi/scsi_mid_low_api.txt
@@ -1343,7 +1343,7 @@ Members of interest:
underruns (overruns should be rare). If possible an LLD
should set 'resid' prior to invoking 'done'. The most
interesting case is data transfers from a SCSI target
- device device (i.e. READs) that underrun.
+ device (e.g. READs) that underrun.
underflow - LLD should place (DID_ERROR << 16) in 'result' if
actual number of bytes transferred is less than this
figure. Not many LLDs implement this check and some that
@@ -1351,6 +1351,18 @@ Members of interest:
report a DID_ERROR. Better for an LLD to implement
+It is recommended that a LLD set 'resid' on data transfers from a SCSI
+target device (e.g. READs). It is especially important that 'resid' is set
+when such data transfers have sense keys of MEDIUM ERROR and HARDWARE ERROR
+(and possibly RECOVERED ERROR). In these cases if a LLD is in doubt how much
+data has been received then the safest approach is to indicate no bytes have
+been received. For example: to indicate that no valid data has been received
+a LLD might use these helpers:
+ scsi_set_resid(SCpnt, scsi_bufflen(SCpnt));
+where 'SCpnt' is a pointer to a scsi_cmnd object. To indicate only three 512
+bytes blocks has been received 'resid' could be set like this:
+ scsi_set_resid(SCpnt, scsi_bufflen(SCpnt) - (3 * 512));
The scsi_cmnd structure is defined in include/scsi/scsi_cmnd.h

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