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diff --git a/Documentation/arm/tcm.rst b/Documentation/arm/tcm.rst deleted file mode 100644 index 1dc6c39220f9..000000000000 --- a/Documentation/arm/tcm.rst +++ /dev/null @@ -1,161 +0,0 @@ -================================================== -ARM TCM (Tightly-Coupled Memory) handling in Linux -================================================== - -Written by Linus Walleij <linus.walleij@stericsson.com> - -Some ARM SoCs 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 CPUs 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 is 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); - } - } |