linux/Documentation/arm/tcm.txt
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   1ARM TCM (Tightly-Coupled Memory) handling in Linux
   2----
   3Written by Linus Walleij <linus.walleij@stericsson.com>
   4
   5Some ARM SoC:s have a so-called TCM (Tightly-Coupled Memory).
   6This is usually just a few (4-64) KiB of RAM inside the ARM
   7processor.
   8
   9Due to being embedded inside the CPU The TCM has a
  10Harvard-architecture, so there is an ITCM (instruction TCM)
  11and a DTCM (data TCM). The DTCM can not contain any
  12instructions, but the ITCM can actually contain data.
  13The size of DTCM or ITCM is minimum 4KiB so the typical
  14minimum configuration is 4KiB ITCM and 4KiB DTCM.
  15
  16ARM CPU:s have special registers to read out status, physical
  17location and size of TCM memories. arch/arm/include/asm/cputype.h
  18defines a CPUID_TCM register that you can read out from the
  19system control coprocessor. Documentation from ARM can be found
  20at http://infocenter.arm.com, search for "TCM Status Register"
  21to see documents for all CPUs. Reading this register you can
  22determine if ITCM (bits 1-0) and/or DTCM (bit 17-16) is present
  23in the machine.
  24
  25There is further a TCM region register (search for "TCM Region
  26Registers" at the ARM site) that can report and modify the location
  27size of TCM memories at runtime. This is used to read out and modify
  28TCM location and size. Notice that this is not a MMU table: you
  29actually move the physical location of the TCM around. At the
  30place you put it, it will mask any underlying RAM from the
  31CPU so it is usually wise not to overlap any physical RAM with
  32the TCM.
  33
  34The TCM memory can then be remapped to another address again using
  35the MMU, but notice that the TCM if often used in situations where
  36the MMU is turned off. To avoid confusion the current Linux
  37implementation will map the TCM 1 to 1 from physical to virtual
  38memory in the location specified by the kernel. Currently Linux
  39will map ITCM to 0xfffe0000 and on, and DTCM to 0xfffe8000 and
  40on, supporting a maximum of 32KiB of ITCM and 32KiB of DTCM.
  41
  42Newer versions of the region registers also support dividing these
  43TCMs in two separate banks, so for example an 8KiB ITCM is divided
  44into two 4KiB banks with its own control registers. The idea is to
  45be able to lock and hide one of the banks for use by the secure
  46world (TrustZone).
  47
  48TCM is used for a few things:
  49
  50- FIQ and other interrupt handlers that need deterministic
  51  timing and cannot wait for cache misses.
  52
  53- Idle loops where all external RAM is set to self-refresh
  54  retention mode, so only on-chip RAM is accessible by
  55  the CPU and then we hang inside ITCM waiting for an
  56  interrupt.
  57
  58- Other operations which implies shutting off or reconfiguring
  59  the external RAM controller.
  60
  61There is an interface for using TCM on the ARM architecture
  62in <asm/tcm.h>. Using this interface it is possible to:
  63
  64- Define the physical address and size of ITCM and DTCM.
  65
  66- Tag functions to be compiled into ITCM.
  67
  68- Tag data and constants to be allocated to DTCM and ITCM.
  69
  70- Have the remaining TCM RAM added to a special
  71  allocation pool with gen_pool_create() and gen_pool_add()
  72  and provice tcm_alloc() and tcm_free() for this
  73  memory. Such a heap is great for things like saving
  74  device state when shutting off device power domains.
  75
  76A machine that has TCM memory shall select HAVE_TCM from
  77arch/arm/Kconfig for itself. Code that needs to use TCM shall
  78#include <asm/tcm.h>
  79
  80Functions to go into itcm can be tagged like this:
  81int __tcmfunc foo(int bar);
  82
  83Since these are marked to become long_calls and you may want
  84to have functions called locally inside the TCM without
  85wasting space, there is also the __tcmlocalfunc prefix that
  86will make the call relative.
  87
  88Variables to go into dtcm can be tagged like this:
  89int __tcmdata foo;
  90
  91Constants can be tagged like this:
  92int __tcmconst foo;
  93
  94To put assembler into TCM just use
  95.section ".tcm.text" or .section ".tcm.data"
  96respectively.
  97
  98Example code:
  99
 100#include <asm/tcm.h>
 101
 102/* Uninitialized data */
 103static u32 __tcmdata tcmvar;
 104/* Initialized data */
 105static u32 __tcmdata tcmassigned = 0x2BADBABEU;
 106/* Constant */
 107static const u32 __tcmconst tcmconst = 0xCAFEBABEU;
 108
 109static void __tcmlocalfunc tcm_to_tcm(void)
 110{
 111        int i;
 112        for (i = 0; i < 100; i++)
 113                tcmvar ++;
 114}
 115
 116static void __tcmfunc hello_tcm(void)
 117{
 118        /* Some abstract code that runs in ITCM */
 119        int i;
 120        for (i = 0; i < 100; i++) {
 121                tcmvar ++;
 122        }
 123        tcm_to_tcm();
 124}
 125
 126static void __init test_tcm(void)
 127{
 128        u32 *tcmem;
 129        int i;
 130
 131        hello_tcm();
 132        printk("Hello TCM executed from ITCM RAM\n");
 133
 134        printk("TCM variable from testrun: %u @ %p\n", tcmvar, &tcmvar);
 135        tcmvar = 0xDEADBEEFU;
 136        printk("TCM variable: 0x%x @ %p\n", tcmvar, &tcmvar);
 137
 138        printk("TCM assigned variable: 0x%x @ %p\n", tcmassigned, &tcmassigned);
 139
 140        printk("TCM constant: 0x%x @ %p\n", tcmconst, &tcmconst);
 141
 142        /* Allocate some TCM memory from the pool */
 143        tcmem = tcm_alloc(20);
 144        if (tcmem) {
 145                printk("TCM Allocated 20 bytes of TCM @ %p\n", tcmem);
 146                tcmem[0] = 0xDEADBEEFU;
 147                tcmem[1] = 0x2BADBABEU;
 148                tcmem[2] = 0xCAFEBABEU;
 149                tcmem[3] = 0xDEADBEEFU;
 150                tcmem[4] = 0x2BADBABEU;
 151                for (i = 0; i < 5; i++)
 152                        printk("TCM tcmem[%d] = %08x\n", i, tcmem[i]);
 153                tcm_free(tcmem, 20);
 154        }
 155}
 156
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