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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
-processor.
-
-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 (bits 1-0) and/or DTCM (bit 17-16) is present
-in the machine.
-
-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 can then be remapped to another address again using
-the MMU, but notice that the TCM if often used in situations where
-the MMU is turned off. To avoid confusion the current Linux
-implementation will map the TCM 1 to 1 from physical to virtual
-memory in the location specified by the kernel. Currently Linux
-will map ITCM to 0xfffe0000 and on, and DTCM to 0xfffe8000 and
-on, supporting a maximum of 32KiB of ITCM and 32KiB of DTCM.
-
-Newer versions of the region registers also support dividing these
-TCMs in two separate banks, so for example an 8KiB ITCM is divided
-into two 4KiB banks with its own control registers. The idea is to
-be able to lock and hide one of the banks for use by the secure
-world (TrustZone).
-
-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 from
-arch/arm/Kconfig for itself. Code that needs to use TCM shall
-#include <asm/tcm.h>
-
-Functions to go into itcm can be tagged like this:
-int __tcmfunc foo(int bar);
-
-Since these are marked to become long_calls and you may want
-to have functions called locally inside the TCM without
-wasting space, there is also the __tcmlocalfunc prefix that
-will make the call relative.
-
-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"
-respectively.
-
-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);
- }
-}