Documentation/arm/tcm.rst

   1 ==================================================
   2 ARM TCM (Tightly-Coupled Memory) handling in Linux
   3 ==================================================
   4
   5 Written by Linus Walleij <linus.walleij@stericsson.com>
   6
   7 Some ARM SoCs have a so-called TCM (Tightly-Coupled Memory).
   8 This is usually just a few (4-64) KiB of RAM inside the ARM
   9 processor.
  10
  11 Due to being embedded inside the CPU, the TCM has a
  12 Harvard-architecture, so there is an ITCM (instruction TCM)
  13 and a DTCM (data TCM). The DTCM can not contain any
  14 instructions, but the ITCM can actually contain data.
  15 The size of DTCM or ITCM is minimum 4KiB so the typical
  16 minimum configuration is 4KiB ITCM and 4KiB DTCM.
  17
  18 ARM CPUs have special registers to read out status, physical
  19 location and size of TCM memories. arch/arm/include/asm/cputype.h
  20 defines a CPUID_TCM register that you can read out from the
  21 system control coprocessor. Documentation from ARM can be found
  22 at http://infocenter.arm.com, search for "TCM Status Register"
  23 to see documents for all CPUs. Reading this register you can
  24 determine if ITCM (bits 1-0) and/or DTCM (bit 17-16) is present
  25 in the machine.
  26
  27 There is further a TCM region register (search for "TCM Region
  28 Registers" at the ARM site) that can report and modify the location
  29 size of TCM memories at runtime. This is used to read out and modify
  30 TCM location and size. Notice that this is not a MMU table: you
  31 actually move the physical location of the TCM around. At the
  32 place you put it, it will mask any underlying RAM from the
  33 CPU so it is usually wise not to overlap any physical RAM with
  34 the TCM.
  35
  36 The TCM memory can then be remapped to another address again using
  37 the MMU, but notice that the TCM is often used in situations where
  38 the MMU is turned off. To avoid confusion the current Linux
  39 implementation will map the TCM 1 to 1 from physical to virtual
  40 memory in the location specified by the kernel. Currently Linux
  41 will map ITCM to 0xfffe0000 and on, and DTCM to 0xfffe8000 and
  42 on, supporting a maximum of 32KiB of ITCM and 32KiB of DTCM.
  43
  44 Newer versions of the region registers also support dividing these
  45 TCMs in two separate banks, so for example an 8KiB ITCM is divided
  46 into two 4KiB banks with its own control registers. The idea is to
  47 be able to lock and hide one of the banks for use by the secure
  48 world (TrustZone).
  49
  50 TCM 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
  63 There is an interface for using TCM on the ARM architecture
  64 in <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
  78 A machine that has TCM memory shall select HAVE_TCM from
  79 arch/arm/Kconfig for itself. Code that needs to use TCM shall
  80 #include <asm/tcm.h>
  81
  82 Functions to go into itcm can be tagged like this:
  83 int __tcmfunc foo(int bar);
  84
  85 Since these are marked to become long_calls and you may want
  86 to have functions called locally inside the TCM without
  87 wasting space, there is also the __tcmlocalfunc prefix that
  88 will make the call relative.
  89
  90 Variables to go into dtcm can be tagged like this::
  91
  92   int __tcmdata foo;
  93
  94 Constants can be tagged like this::
  95
  96   int __tcmconst foo;
  97
  98 To put assembler into TCM just use::
  99
 100   .section ".tcm.text" or .section ".tcm.data"
 101
 102 respectively.
 103
 104 Example 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   }