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This changes the TCM handling so that a fixed area is reserved at 0xfffe0000-0xfffeffff for TCM. This areas is used by XScale but XScale does not have TCM so the mechanisms are mutually exclusive. This change is needed to make TCM detection more dynamic while still being able to compile code into it, and is a must for the unified ARM goals: the current TCM allocation at different places in memory for each machine would be a nightmare if you want to compile a single image for more than one machine with TCM so it has to be nailed down in one place. Signed-off-by: Linus Walleij <linus.walleij@stericsson.com> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
155 lines
5 KiB
Text
155 lines
5 KiB
Text
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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