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. The TCM memory exists totally outside the MMU and will
  33 override any MMU mappings.
  34
  35 Code executing inside the ITCM does not "see" any MMU mappings
  36 and e.g. register accesses must be made to physical addresses.
  37
  38 TCM is used for a few things:
  39
  40 - FIQ and other interrupt handlers that need deterministic
  41   timing and cannot wait for cache misses.
  42
  43 - Idle loops where all external RAM is set to self-refresh
  44   retention mode, so only on-chip RAM is accessible by
  45   the CPU and then we hang inside ITCM waiting for an
  46   interrupt.
  47
  48 - Other operations which implies shutting off or reconfiguring
  49   the external RAM controller.
  50
  51 There is an interface for using TCM on the ARM architecture
  52 in <asm/tcm.h>. Using this interface it is possible to:
  53
  54 - Define the physical address and size of ITCM and DTCM.
  55
  56 - Tag functions to be compiled into ITCM.
  57
  58 - Tag data and constants to be allocated to DTCM and ITCM.
  59
  60 - Have the remaining TCM RAM added to a special
  61   allocation pool with gen_pool_create() and gen_pool_add()
  62   and provice tcm_alloc() and tcm_free() for this
  63   memory. Such a heap is great for things like saving
  64   device state when shutting off device power domains.
  65
  66 A machine that has TCM memory shall select HAVE_TCM in
  67 arch/arm/Kconfig for itself, and then the
  68 rest of the functionality will depend on the physical
  69 location and size of ITCM and DTCM to be defined in
  70 mach/memory.h for the machine. Code that needs to use
  71 TCM shall #include <asm/tcm.h> If the TCM is not located
  72 at the place given in memory.h it will be moved using
  73 the TCM Region registers.
  74
  75 Functions to go into itcm can be tagged like this:
  76 int __tcmfunc foo(int bar);
  77
  78 Variables to go into dtcm can be tagged like this:
  79 int __tcmdata foo;
  80
  81 Constants can be tagged like this:
  82 int __tcmconst foo;
  83
  84 To put assembler into TCM just use
  85 .section ".tcm.text" or .section ".tcm.data"
  86 respectively.
  87
  88 Example code:
  89
  90 #include <asm/tcm.h>
  91
  92 /* Uninitialized data */
  93 static u32 __tcmdata tcmvar;
  94 /* Initialized data */
  95 static u32 __tcmdata tcmassigned = 0x2BADBABEU;
  96 /* Constant */
  97 static const u32 __tcmconst tcmconst = 0xCAFEBABEU;
  98
  99 static void __tcmlocalfunc tcm_to_tcm(void)
 100 {
 101         int i;
 102         for (i = 0; i < 100; i++)
 103                 tcmvar ++;
 104 }
 105
 106 static void __tcmfunc hello_tcm(void)
 107 {
 108         /* Some abstract code that runs in ITCM */
 109         int i;
 110         for (i = 0; i < 100; i++) {
 111                 tcmvar ++;
 112         }
 113         tcm_to_tcm();
 114 }
 115
 116 static void __init test_tcm(void)
 117 {
 118         u32 *tcmem;
 119         int i;
 120
 121         hello_tcm();
 122         printk("Hello TCM executed from ITCM RAM\n");
 123
 124         printk("TCM variable from testrun: %u @ %p\n", tcmvar, &tcmvar);
 125         tcmvar = 0xDEADBEEFU;
 126         printk("TCM variable: 0x%x @ %p\n", tcmvar, &tcmvar);
 127
 128         printk("TCM assigned variable: 0x%x @ %p\n", tcmassigned, &tcmassigned);
 129
 130         printk("TCM constant: 0x%x @ %p\n", tcmconst, &tcmconst);
 131
 132         /* Allocate some TCM memory from the pool */
 133         tcmem = tcm_alloc(20);
 134         if (tcmem) {
 135                 printk("TCM Allocated 20 bytes of TCM @ %p\n", tcmem);
 136                 tcmem[0] = 0xDEADBEEFU;
 137                 tcmem[1] = 0x2BADBABEU;
 138                 tcmem[2] = 0xCAFEBABEU;
 139                 tcmem[3] = 0xDEADBEEFU;
 140                 tcmem[4] = 0x2BADBABEU;
 141                 for (i = 0; i < 5; i++)
 142                         printk("TCM tcmem[%d] = %08x\n", i, tcmem[i]);
 143                 tcm_free(tcmem, 20);
 144         }
 145 }