path: root/Documentation/arm
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authorLinus Walleij <linus.walleij@stericsson.com>2009-09-15 16:30:37 (GMT)
committerRussell King <rmk+kernel@arm.linux.org.uk>2009-09-15 21:11:05 (GMT)
commitbc581770cfdd8c17ea17d324dc05e2f9c599e7ca (patch)
treec6d2c2eb82d8a6c6851de0ecc7d8c0e63e026266 /Documentation/arm
parent18240904960a39e582ced8ba8ececb10b8c22dd3 (diff)
ARM: 5580/2: ARM TCM (Tightly-Coupled Memory) support v3
This adds the TCM interface to Linux, when active, it will detect and report TCM memories and sizes early in boot if present, introduce generic TCM memory handling, provide a generic TCM memory pool and select TCM memory for the U300 platform. See the Documentation/arm/tcm.txt for documentation. Signed-off-by: Linus Walleij <linus.walleij@stericsson.com> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
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+ARM TCM (Tightly-Coupled Memory) handling in Linux
+Written by Linus Walleij <linus.walleij@stericsson.com>
+Some ARM SoC:s have a so-called TCM (Tightly-Coupled Memory).
+This is usually just a few (4-64) KiB of RAM inside the ARM
+Due to being embedded inside the CPU The TCM has a
+Harvard-architecture, so there is an ITCM (instruction TCM)
+and a DTCM (data TCM). The DTCM can not contain any
+instructions, but the ITCM can actually contain data.
+The size of DTCM or ITCM is minimum 4KiB so the typical
+minimum configuration is 4KiB ITCM and 4KiB DTCM.
+ARM CPU:s have special registers to read out status, physical
+location and size of TCM memories. arch/arm/include/asm/cputype.h
+defines a CPUID_TCM register that you can read out from the
+system control coprocessor. Documentation from ARM can be found
+at http://infocenter.arm.com, search for "TCM Status Register"
+to see documents for all CPUs. Reading this register you can
+determine if ITCM (bit 0) and/or DTCM (bit 16) is present in the
+There is further a TCM region register (search for "TCM Region
+Registers" at the ARM site) that can report and modify the location
+size of TCM memories at runtime. This is used to read out and modify
+TCM location and size. Notice that this is not a MMU table: you
+actually move the physical location of the TCM around. At the
+place you put it, it will mask any underlying RAM from the
+CPU so it is usually wise not to overlap any physical RAM with
+the TCM. The TCM memory exists totally outside the MMU and will
+override any MMU mappings.
+Code executing inside the ITCM does not "see" any MMU mappings
+and e.g. register accesses must be made to physical addresses.
+TCM is used for a few things:
+- FIQ and other interrupt handlers that need deterministic
+ timing and cannot wait for cache misses.
+- Idle loops where all external RAM is set to self-refresh
+ retention mode, so only on-chip RAM is accessible by
+ the CPU and then we hang inside ITCM waiting for an
+ interrupt.
+- Other operations which implies shutting off or reconfiguring
+ the external RAM controller.
+There is an interface for using TCM on the ARM architecture
+in <asm/tcm.h>. Using this interface it is possible to:
+- Define the physical address and size of ITCM and DTCM.
+- Tag functions to be compiled into ITCM.
+- Tag data and constants to be allocated to DTCM and ITCM.
+- Have the remaining TCM RAM added to a special
+ allocation pool with gen_pool_create() and gen_pool_add()
+ and provice tcm_alloc() and tcm_free() for this
+ memory. Such a heap is great for things like saving
+ device state when shutting off device power domains.
+A machine that has TCM memory shall select HAVE_TCM in
+arch/arm/Kconfig for itself, and then the
+rest of the functionality will depend on the physical
+location and size of ITCM and DTCM to be defined in
+mach/memory.h for the machine. Code that needs to use
+TCM shall #include <asm/tcm.h> If the TCM is not located
+at the place given in memory.h it will be moved using
+the TCM Region registers.
+Functions to go into itcm can be tagged like this:
+int __tcmfunc foo(int bar);
+Variables to go into dtcm can be tagged like this:
+int __tcmdata foo;
+Constants can be tagged like this:
+int __tcmconst foo;
+To put assembler into TCM just use
+.section ".tcm.text" or .section ".tcm.data"
+Example code:
+#include <asm/tcm.h>
+/* Uninitialized data */
+static u32 __tcmdata tcmvar;
+/* Initialized data */
+static u32 __tcmdata tcmassigned = 0x2BADBABEU;
+/* Constant */
+static const u32 __tcmconst tcmconst = 0xCAFEBABEU;
+static void __tcmlocalfunc tcm_to_tcm(void)
+ int i;
+ for (i = 0; i < 100; i++)
+ tcmvar ++;
+static void __tcmfunc hello_tcm(void)
+ /* Some abstract code that runs in ITCM */
+ int i;
+ for (i = 0; i < 100; i++) {
+ tcmvar ++;
+ }
+ tcm_to_tcm();
+static void __init test_tcm(void)
+ u32 *tcmem;
+ int i;
+ hello_tcm();
+ printk("Hello TCM executed from ITCM RAM\n");
+ printk("TCM variable from testrun: %u @ %p\n", tcmvar, &tcmvar);
+ tcmvar = 0xDEADBEEFU;
+ printk("TCM variable: 0x%x @ %p\n", tcmvar, &tcmvar);
+ printk("TCM assigned variable: 0x%x @ %p\n", tcmassigned, &tcmassigned);
+ printk("TCM constant: 0x%x @ %p\n", tcmconst, &tcmconst);
+ /* Allocate some TCM memory from the pool */
+ tcmem = tcm_alloc(20);
+ if (tcmem) {
+ printk("TCM Allocated 20 bytes of TCM @ %p\n", tcmem);
+ tcmem[0] = 0xDEADBEEFU;
+ tcmem[1] = 0x2BADBABEU;
+ tcmem[2] = 0xCAFEBABEU;
+ tcmem[3] = 0xDEADBEEFU;
+ tcmem[4] = 0x2BADBABEU;
+ for (i = 0; i < 5; i++)
+ printk("TCM tcmem[%d] = %08x\n", i, tcmem[i]);
+ tcm_free(tcmem, 20);
+ }

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