1ARM TCM (Tightly-Coupled Memory) handling in Linux 2---- 3Written by Linus Walleij <linus.walleij@stericsson.com> 4 5Some ARM SoC:s have a so-called TCM (Tightly-Coupled Memory). 6This is usually just a few (4-64) KiB of RAM inside the ARM 7processor. 8 9Due to being embedded inside the CPU The TCM has a 10Harvard-architecture, so there is an ITCM (instruction TCM) 11and a DTCM (data TCM). The DTCM can not contain any 12instructions, but the ITCM can actually contain data. 13The size of DTCM or ITCM is minimum 4KiB so the typical 14minimum configuration is 4KiB ITCM and 4KiB DTCM. 15 16ARM CPU:s have special registers to read out status, physical 17location and size of TCM memories. arch/arm/include/asm/cputype.h 18defines a CPUID_TCM register that you can read out from the 19system control coprocessor. Documentation from ARM can be found 20at http://infocenter.arm.com, search for "TCM Status Register" 21to see documents for all CPUs. Reading this register you can 22determine if ITCM (bits 1-0) and/or DTCM (bit 17-16) is present 23in the machine. 24 25There is further a TCM region register (search for "TCM Region 26Registers" at the ARM site) that can report and modify the location 27size of TCM memories at runtime. This is used to read out and modify 28TCM location and size. Notice that this is not a MMU table: you 29actually move the physical location of the TCM around. At the 30place you put it, it will mask any underlying RAM from the 31CPU so it is usually wise not to overlap any physical RAM with 32the TCM. 33 34The TCM memory can then be remapped to another address again using 35the MMU, but notice that the TCM if often used in situations where 36the MMU is turned off. To avoid confusion the current Linux 37implementation will map the TCM 1 to 1 from physical to virtual 38memory in the location specified by the kernel. Currently Linux 39will map ITCM to 0xfffe0000 and on, and DTCM to 0xfffe8000 and 40on, supporting a maximum of 32KiB of ITCM and 32KiB of DTCM. 41 42Newer versions of the region registers also support dividing these 43TCMs in two separate banks, so for example an 8KiB ITCM is divided 44into two 4KiB banks with its own control registers. The idea is to 45be able to lock and hide one of the banks for use by the secure 46world (TrustZone). 47 48TCM 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 61There is an interface for using TCM on the ARM architecture 62in <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 76A machine that has TCM memory shall select HAVE_TCM from 77arch/arm/Kconfig for itself. Code that needs to use TCM shall 78#include <asm/tcm.h> 79 80Functions to go into itcm can be tagged like this: 81int __tcmfunc foo(int bar); 82 83Since these are marked to become long_calls and you may want 84to have functions called locally inside the TCM without 85wasting space, there is also the __tcmlocalfunc prefix that 86will make the call relative. 87 88Variables to go into dtcm can be tagged like this: 89int __tcmdata foo; 90 91Constants can be tagged like this: 92int __tcmconst foo; 93 94To put assembler into TCM just use 95.section ".tcm.text" or .section ".tcm.data" 96respectively. 97 98Example code: 99 100#include <asm/tcm.h> 101 102/* Uninitialized data */ 103static u32 __tcmdata tcmvar; 104/* Initialized data */ 105static u32 __tcmdata tcmassigned = 0x2BADBABEU; 106/* Constant */ 107static const u32 __tcmconst tcmconst = 0xCAFEBABEU; 108 109static void __tcmlocalfunc tcm_to_tcm(void) 110{ 111 int i; 112 for (i = 0; i < 100; i++) 113 tcmvar ++; 114} 115 116static 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 126static 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} 156