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 (bit 0) and/or DTCM (bit 16) is present in the
  23 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 machine.
  39
  40 TCM is used for a few things:
  41
  42 - FIQ and other interrupt handlers that need deterministic
  43   timing and cannot wait for cache misses.
  44
  45 - Idle loops where all external RAM is set to self-refresh
  46   retention mode, so only on-chip RAM is accessible by
  47   the CPU and then we hang inside ITCM waiting for an
  48   interrupt.
  49
  50 - Other operations which implies shutting off or reconfiguring
  51   the external RAM controller.
  52
  53 There is an interface for using TCM on the ARM architecture
  54 in <asm/tcm.h>. Using this interface it is possible to:
  55
  56 - Define the physical address and size of ITCM and DTCM.
  57
  58 - Tag functions to be compiled into ITCM.
  59
  60 - Tag data and constants to be allocated to DTCM and ITCM.
  61
  62 - Have the remaining TCM RAM added to a special
  63   allocation pool with gen_pool_create() and gen_pool_add()
  64   and provice tcm_alloc() and tcm_free() for this
  65   memory. Such a heap is great for things like saving
  66   device state when shutting off device power domains.
  67
  68 A machine that has TCM memory shall select HAVE_TCM in
  69 arch/arm/Kconfig for itself, and then the
  70 rest of the functionality will depend on the physical
  71 location and size of ITCM and DTCM to be defined in
  72 mach/memory.h for the machine. Code that needs to use
  73 TCM shall #include <asm/tcm.h> If the TCM is not located
  74 at the place given in memory.h it will be moved using
  75 the TCM Region registers.
  76
  77 Functions to go into itcm can be tagged like this:
  78 int __tcmfunc foo(int bar);
  79
  80 Variables to go into dtcm can be tagged like this:
  81 int __tcmdata foo;
  82
  83 Constants can be tagged like this:
  84 int __tcmconst foo;
  85
  86 To put assembler into TCM just use
  87 .section ".tcm.text" or .section ".tcm.data"
  88 respectively.
  89
  90 Example code:
  91
  92 #include <asm/tcm.h>
  93
  94 /* Uninitialized data */
  95 static u32 __tcmdata tcmvar;
  96 /* Initialized data */
  97 static u32 __tcmdata tcmassigned = 0x2BADBABEU;
  98 /* Constant */
  99 static const u32 __tcmconst tcmconst = 0xCAFEBABEU;
 100
 101 static void __tcmlocalfunc tcm_to_tcm(void)
 102 {
 103         int i;
 104         for (i = 0; i < 100; i++)
 105                 tcmvar ++;
 106 }
 107
 108 static void __tcmfunc hello_tcm(void)
 109 {
 110         /* Some abstract code that runs in ITCM */
 111         int i;
 112         for (i = 0; i < 100; i++) {
 113                 tcmvar ++;
 114         }
 115         tcm_to_tcm();
 116 }
 117
 118 static void __init test_tcm(void)
 119 {
 120         u32 *tcmem;
 121         int i;
 122
 123         hello_tcm();
 124         printk("Hello TCM executed from ITCM RAM\n");
 125
 126         printk("TCM variable from testrun: %u @ %p\n", tcmvar, &tcmvar);
 127         tcmvar = 0xDEADBEEFU;
 128         printk("TCM variable: 0x%x @ %p\n", tcmvar, &tcmvar);
 129
 130         printk("TCM assigned variable: 0x%x @ %p\n", tcmassigned, &tcmassigned);
 131
 132         printk("TCM constant: 0x%x @ %p\n", tcmconst, &tcmconst);
 133
 134         /* Allocate some TCM memory from the pool */
 135         tcmem = tcm_alloc(20);
 136         if (tcmem) {
 137                 printk("TCM Allocated 20 bytes of TCM @ %p\n", tcmem);
 138                 tcmem[0] = 0xDEADBEEFU;
 139                 tcmem[1] = 0x2BADBABEU;
 140                 tcmem[2] = 0xCAFEBABEU;
 141                 tcmem[3] = 0xDEADBEEFU;
 142                 tcmem[4] = 0x2BADBABEU;
 143                 for (i = 0; i < 5; i++)
 144                         printk("TCM tcmem[%d] = %08x\n", i, tcmem[i]);
 145                 tcm_free(tcmem, 20);
 146         }
 147 }