linux/Documentation/arm/tcm.rst
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   2ARM TCM (Tightly-Coupled Memory) handling in Linux
   3==================================================
   4
   5Written by Linus Walleij <linus.walleij@stericsson.com>
   6
   7Some ARM SoCs have a so-called TCM (Tightly-Coupled Memory).
   8This is usually just a few (4-64) KiB of RAM inside the ARM
   9processor.
  10
  11Due to being embedded inside the CPU, the TCM has a
  12Harvard-architecture, so there is an ITCM (instruction TCM)
  13and a DTCM (data TCM). The DTCM can not contain any
  14instructions, but the ITCM can actually contain data.
  15The size of DTCM or ITCM is minimum 4KiB so the typical
  16minimum configuration is 4KiB ITCM and 4KiB DTCM.
  17
  18ARM CPUs have special registers to read out status, physical
  19location and size of TCM memories. arch/arm/include/asm/cputype.h
  20defines a CPUID_TCM register that you can read out from the
  21system control coprocessor. Documentation from ARM can be found
  22at http://infocenter.arm.com, search for "TCM Status Register"
  23to see documents for all CPUs. Reading this register you can
  24determine if ITCM (bits 1-0) and/or DTCM (bit 17-16) is present
  25in the machine.
  26
  27There is further a TCM region register (search for "TCM Region
  28Registers" at the ARM site) that can report and modify the location
  29size of TCM memories at runtime. This is used to read out and modify
  30TCM location and size. Notice that this is not a MMU table: you
  31actually move the physical location of the TCM around. At the
  32place you put it, it will mask any underlying RAM from the
  33CPU so it is usually wise not to overlap any physical RAM with
  34the TCM.
  35
  36The TCM memory can then be remapped to another address again using
  37the MMU, but notice that the TCM if often used in situations where
  38the MMU is turned off. To avoid confusion the current Linux
  39implementation will map the TCM 1 to 1 from physical to virtual
  40memory in the location specified by the kernel. Currently Linux
  41will map ITCM to 0xfffe0000 and on, and DTCM to 0xfffe8000 and
  42on, supporting a maximum of 32KiB of ITCM and 32KiB of DTCM.
  43
  44Newer versions of the region registers also support dividing these
  45TCMs in two separate banks, so for example an 8KiB ITCM is divided
  46into two 4KiB banks with its own control registers. The idea is to
  47be able to lock and hide one of the banks for use by the secure
  48world (TrustZone).
  49
  50TCM is used for a few things:
  51
  52- FIQ and other interrupt handlers that need deterministic
  53  timing and cannot wait for cache misses.
  54
  55- Idle loops where all external RAM is set to self-refresh
  56  retention mode, so only on-chip RAM is accessible by
  57  the CPU and then we hang inside ITCM waiting for an
  58  interrupt.
  59
  60- Other operations which implies shutting off or reconfiguring
  61  the external RAM controller.
  62
  63There is an interface for using TCM on the ARM architecture
  64in <asm/tcm.h>. Using this interface it is possible to:
  65
  66- Define the physical address and size of ITCM and DTCM.
  67
  68- Tag functions to be compiled into ITCM.
  69
  70- Tag data and constants to be allocated to DTCM and ITCM.
  71
  72- Have the remaining TCM RAM added to a special
  73  allocation pool with gen_pool_create() and gen_pool_add()
  74  and provice tcm_alloc() and tcm_free() for this
  75  memory. Such a heap is great for things like saving
  76  device state when shutting off device power domains.
  77
  78A machine that has TCM memory shall select HAVE_TCM from
  79arch/arm/Kconfig for itself. Code that needs to use TCM shall
  80#include <asm/tcm.h>
  81
  82Functions to go into itcm can be tagged like this:
  83int __tcmfunc foo(int bar);
  84
  85Since these are marked to become long_calls and you may want
  86to have functions called locally inside the TCM without
  87wasting space, there is also the __tcmlocalfunc prefix that
  88will make the call relative.
  89
  90Variables to go into dtcm can be tagged like this::
  91
  92  int __tcmdata foo;
  93
  94Constants can be tagged like this::
  95
  96  int __tcmconst foo;
  97
  98To put assembler into TCM just use::
  99
 100  .section ".tcm.text" or .section ".tcm.data"
 101
 102respectively.
 103
 104Example code::
 105
 106  #include <asm/tcm.h>
 107
 108  /* Uninitialized data */
 109  static u32 __tcmdata tcmvar;
 110  /* Initialized data */
 111  static u32 __tcmdata tcmassigned = 0x2BADBABEU;
 112  /* Constant */
 113  static const u32 __tcmconst tcmconst = 0xCAFEBABEU;
 114
 115  static void __tcmlocalfunc tcm_to_tcm(void)
 116  {
 117        int i;
 118        for (i = 0; i < 100; i++)
 119                tcmvar ++;
 120  }
 121
 122  static void __tcmfunc hello_tcm(void)
 123  {
 124        /* Some abstract code that runs in ITCM */
 125        int i;
 126        for (i = 0; i < 100; i++) {
 127                tcmvar ++;
 128        }
 129        tcm_to_tcm();
 130  }
 131
 132  static void __init test_tcm(void)
 133  {
 134        u32 *tcmem;
 135        int i;
 136
 137        hello_tcm();
 138        printk("Hello TCM executed from ITCM RAM\n");
 139
 140        printk("TCM variable from testrun: %u @ %p\n", tcmvar, &tcmvar);
 141        tcmvar = 0xDEADBEEFU;
 142        printk("TCM variable: 0x%x @ %p\n", tcmvar, &tcmvar);
 143
 144        printk("TCM assigned variable: 0x%x @ %p\n", tcmassigned, &tcmassigned);
 145
 146        printk("TCM constant: 0x%x @ %p\n", tcmconst, &tcmconst);
 147
 148        /* Allocate some TCM memory from the pool */
 149        tcmem = tcm_alloc(20);
 150        if (tcmem) {
 151                printk("TCM Allocated 20 bytes of TCM @ %p\n", tcmem);
 152                tcmem[0] = 0xDEADBEEFU;
 153                tcmem[1] = 0x2BADBABEU;
 154                tcmem[2] = 0xCAFEBABEU;
 155                tcmem[3] = 0xDEADBEEFU;
 156                tcmem[4] = 0x2BADBABEU;
 157                for (i = 0; i < 5; i++)
 158                        printk("TCM tcmem[%d] = %08x\n", i, tcmem[i]);
 159                tcm_free(tcmem, 20);
 160        }
 161  }
 162