156 lines
5 KiB
Plaintext
156 lines
5 KiB
Plaintext
ARM TCM (Tightly-Coupled Memory) handling in Linux
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----
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Written by Linus Walleij <linus.walleij@stericsson.com>
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Some ARM SoC:s have a so-called TCM (Tightly-Coupled Memory).
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This is usually just a few (4-64) KiB of RAM inside the ARM
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processor.
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Due to being embedded inside the CPU The TCM has a
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Harvard-architecture, so there is an ITCM (instruction TCM)
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and a DTCM (data TCM). The DTCM can not contain any
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instructions, but the ITCM can actually contain data.
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The size of DTCM or ITCM is minimum 4KiB so the typical
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minimum configuration is 4KiB ITCM and 4KiB DTCM.
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ARM CPU:s have special registers to read out status, physical
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location and size of TCM memories. arch/arm/include/asm/cputype.h
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defines a CPUID_TCM register that you can read out from the
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system control coprocessor. Documentation from ARM can be found
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at http://infocenter.arm.com, search for "TCM Status Register"
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to see documents for all CPUs. Reading this register you can
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determine if ITCM (bits 1-0) and/or DTCM (bit 17-16) is present
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in the machine.
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There is further a TCM region register (search for "TCM Region
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Registers" at the ARM site) that can report and modify the location
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size of TCM memories at runtime. This is used to read out and modify
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TCM location and size. Notice that this is not a MMU table: you
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actually move the physical location of the TCM around. At the
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place you put it, it will mask any underlying RAM from the
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CPU so it is usually wise not to overlap any physical RAM with
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the TCM.
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The TCM memory can then be remapped to another address again using
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the MMU, but notice that the TCM if often used in situations where
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the MMU is turned off. To avoid confusion the current Linux
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implementation will map the TCM 1 to 1 from physical to virtual
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memory in the location specified by the kernel. Currently Linux
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will map ITCM to 0xfffe0000 and on, and DTCM to 0xfffe8000 and
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on, supporting a maximum of 32KiB of ITCM and 32KiB of DTCM.
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Newer versions of the region registers also support dividing these
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TCMs in two separate banks, so for example an 8KiB ITCM is divided
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into two 4KiB banks with its own control registers. The idea is to
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be able to lock and hide one of the banks for use by the secure
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world (TrustZone).
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TCM is used for a few things:
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- FIQ and other interrupt handlers that need deterministic
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timing and cannot wait for cache misses.
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- Idle loops where all external RAM is set to self-refresh
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retention mode, so only on-chip RAM is accessible by
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the CPU and then we hang inside ITCM waiting for an
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interrupt.
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- Other operations which implies shutting off or reconfiguring
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the external RAM controller.
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There is an interface for using TCM on the ARM architecture
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in <asm/tcm.h>. Using this interface it is possible to:
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- Define the physical address and size of ITCM and DTCM.
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- Tag functions to be compiled into ITCM.
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- Tag data and constants to be allocated to DTCM and ITCM.
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- Have the remaining TCM RAM added to a special
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allocation pool with gen_pool_create() and gen_pool_add()
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and provice tcm_alloc() and tcm_free() for this
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memory. Such a heap is great for things like saving
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device state when shutting off device power domains.
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A machine that has TCM memory shall select HAVE_TCM from
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arch/arm/Kconfig for itself. Code that needs to use TCM shall
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#include <asm/tcm.h>
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Functions to go into itcm can be tagged like this:
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int __tcmfunc foo(int bar);
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Since these are marked to become long_calls and you may want
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to have functions called locally inside the TCM without
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wasting space, there is also the __tcmlocalfunc prefix that
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will make the call relative.
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Variables to go into dtcm can be tagged like this:
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int __tcmdata foo;
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Constants can be tagged like this:
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int __tcmconst foo;
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To put assembler into TCM just use
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.section ".tcm.text" or .section ".tcm.data"
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respectively.
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Example code:
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#include <asm/tcm.h>
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/* Uninitialized data */
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static u32 __tcmdata tcmvar;
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/* Initialized data */
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static u32 __tcmdata tcmassigned = 0x2BADBABEU;
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/* Constant */
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static const u32 __tcmconst tcmconst = 0xCAFEBABEU;
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static void __tcmlocalfunc tcm_to_tcm(void)
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{
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int i;
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for (i = 0; i < 100; i++)
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tcmvar ++;
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}
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static void __tcmfunc hello_tcm(void)
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{
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/* Some abstract code that runs in ITCM */
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int i;
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for (i = 0; i < 100; i++) {
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tcmvar ++;
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}
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tcm_to_tcm();
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}
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static void __init test_tcm(void)
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{
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u32 *tcmem;
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int i;
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hello_tcm();
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printk("Hello TCM executed from ITCM RAM\n");
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printk("TCM variable from testrun: %u @ %p\n", tcmvar, &tcmvar);
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tcmvar = 0xDEADBEEFU;
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printk("TCM variable: 0x%x @ %p\n", tcmvar, &tcmvar);
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printk("TCM assigned variable: 0x%x @ %p\n", tcmassigned, &tcmassigned);
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printk("TCM constant: 0x%x @ %p\n", tcmconst, &tcmconst);
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/* Allocate some TCM memory from the pool */
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tcmem = tcm_alloc(20);
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if (tcmem) {
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printk("TCM Allocated 20 bytes of TCM @ %p\n", tcmem);
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tcmem[0] = 0xDEADBEEFU;
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tcmem[1] = 0x2BADBABEU;
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tcmem[2] = 0xCAFEBABEU;
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tcmem[3] = 0xDEADBEEFU;
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tcmem[4] = 0x2BADBABEU;
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for (i = 0; i < 5; i++)
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printk("TCM tcmem[%d] = %08x\n", i, tcmem[i]);
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tcm_free(tcmem, 20);
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}
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}
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