Documentation/arm/tcm.txt

   1 ARM TCM (Tightly-Coupled Memory) handling in Linux
   2 ----
   3 Written by Linus Walleij <linus.walleij@stericsson.com>
   4
   5 Some ARM SoC:s have a so-called TCM (Tightly-Coupled Memory).
   6 This is usually just a few (4-64) KiB of RAM inside the ARM
   7 processor.
   8
   9 Due to being embedded inside the CPU The TCM has a
  10 Harvard-architecture, so there is an ITCM (instruction TCM)
  11 and a DTCM (data TCM). The DTCM can not contain any
  12 instructions, but the ITCM can actually contain data.
  13 The size of DTCM or ITCM is minimum 4KiB so the typical
  14 minimum configuration is 4KiB ITCM and 4KiB DTCM.
  15
  16 ARM CPU:s have special registers to read out status, physical
  17 location and size of TCM memories. arch/arm/include/asm/cputype.h
  18 defines a CPUID_TCM register that you can read out from the
  19 system control coprocessor. Documentation from ARM can be found
  20 at http://infocenter.arm.com, search for "TCM Status Register"
  21 to see documents for all CPUs. Reading this register you can
  22 determine if ITCM (bits 1-0) and/or DTCM (bit 17-16) is present
  23 in the machine.
  24
  25 There is further a TCM region register (search for "TCM Region
  26 Registers" at the ARM site) that can report and modify the location
  27 size of TCM memories at runtime. This is used to read out and modify
  28 TCM location and size. Notice that this is not a MMU table: you
  29 actually move the physical location of the TCM around. At the
  30 place you put it, it will mask any underlying RAM from the
  31 CPU so it is usually wise not to overlap any physical RAM with
  32 the TCM.
  33
  34 The TCM memory can then be remapped to another address again using
  35 the MMU, but notice that the TCM if often used in situations where
  36 the MMU is turned off. To avoid confusion the current Linux
  37 implementation will map the TCM 1 to 1 from physical to virtual
  38 memory in the location specified by the kernel. Currently Linux
  39 will map ITCM to 0xfffe0000 and on, and DTCM to 0xfffe8000 and
  40 on, supporting a maximum of 32KiB of ITCM and 32KiB of DTCM.
  41
  42 Newer versions of the region registers also support dividing these
  43 TCMs in two separate banks, so for example an 8KiB ITCM is divided
  44 into two 4KiB banks with its own control registers. The idea is to
  45 be able to lock and hide one of the banks for use by the secure
  46 world (TrustZone).
  47
  48 TCM 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
  61 There is an interface for using TCM on the ARM architecture
  62 in <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
  76 A machine that has TCM memory shall select HAVE_TCM from
  77 arch/arm/Kconfig for itself. Code that needs to use TCM shall
  78 #include <asm/tcm.h>
  79
  80 Functions to go into itcm can be tagged like this:
  81 int __tcmfunc foo(int bar);
  82
  83 Since these are marked to become long_calls and you may want
  84 to have functions called locally inside the TCM without
  85 wasting space, there is also the __tcmlocalfunc prefix that
  86 will make the call relative.
  87
  88 Variables to go into dtcm can be tagged like this:
  89 int __tcmdata foo;
  90
  91 Constants can be tagged like this:
  92 int __tcmconst foo;
  93
  94 To put assembler into TCM just use
  95 .section ".tcm.text" or .section ".tcm.data"
  96 respectively.
  97
  98 Example code:
  99
 100 #include <asm/tcm.h>
 101
 102 /* Uninitialized data */
 103 static u32 __tcmdata tcmvar;
 104 /* Initialized data */
 105 static u32 __tcmdata tcmassigned = 0x2BADBABEU;
 106 /* Constant */
 107 static const u32 __tcmconst tcmconst = 0xCAFEBABEU;
 108
 109 static void __tcmlocalfunc tcm_to_tcm(void)
 110 {
 111         int i;
 112         for (i = 0; i < 100; i++)
 113                 tcmvar ++;
 114 }
 115
 116 static 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
 126 static 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 }