1================================================== 2ARM TCM (Tightly-Coupled Memory) handling in Linux 3================================================== 4 5Written by Linus Walleij <linus.walleij@stericsson.com> 6 7Some ARM SoCs have a so-called TCM (Tightly-Coupled Memory). 8This is usually just a few (4-64) KiB of RAM inside the ARM 9processor. 10 11Due to being embedded inside the CPU, the TCM has a 12Harvard-architecture, so there is an ITCM (instruction TCM) 13and a DTCM (data TCM). The DTCM can not contain any 14instructions, but the ITCM can actually contain data. 15The size of DTCM or ITCM is minimum 4KiB so the typical 16minimum configuration is 4KiB ITCM and 4KiB DTCM. 17 18ARM CPUs have special registers to read out status, physical 19location and size of TCM memories. arch/arm/include/asm/cputype.h 20defines a CPUID_TCM register that you can read out from the 21system control coprocessor. Documentation from ARM can be found 22at http://infocenter.arm.com, search for "TCM Status Register" 23to see documents for all CPUs. Reading this register you can 24determine if ITCM (bits 1-0) and/or DTCM (bit 17-16) is present 25in the machine. 26 27There is further a TCM region register (search for "TCM Region 28Registers" at the ARM site) that can report and modify the location 29size of TCM memories at runtime. This is used to read out and modify 30TCM location and size. Notice that this is not a MMU table: you 31actually move the physical location of the TCM around. At the 32place you put it, it will mask any underlying RAM from the 33CPU so it is usually wise not to overlap any physical RAM with 34the TCM. 35 36The TCM memory can then be remapped to another address again using 37the MMU, but notice that the TCM is often used in situations where 38the MMU is turned off. To avoid confusion the current Linux 39implementation will map the TCM 1 to 1 from physical to virtual 40memory in the location specified by the kernel. Currently Linux 41will map ITCM to 0xfffe0000 and on, and DTCM to 0xfffe8000 and 42on, supporting a maximum of 32KiB of ITCM and 32KiB of DTCM. 43 44Newer versions of the region registers also support dividing these 45TCMs in two separate banks, so for example an 8KiB ITCM is divided 46into two 4KiB banks with its own control registers. The idea is to 47be able to lock and hide one of the banks for use by the secure 48world (TrustZone). 49 50TCM 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 63There is an interface for using TCM on the ARM architecture 64in <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 78A machine that has TCM memory shall select HAVE_TCM from 79arch/arm/Kconfig for itself. Code that needs to use TCM shall 80#include <asm/tcm.h> 81 82Functions to go into itcm can be tagged like this: 83int __tcmfunc foo(int bar); 84 85Since these are marked to become long_calls and you may want 86to have functions called locally inside the TCM without 87wasting space, there is also the __tcmlocalfunc prefix that 88will make the call relative. 89 90Variables to go into dtcm can be tagged like this:: 91 92 int __tcmdata foo; 93 94Constants can be tagged like this:: 95 96 int __tcmconst foo; 97 98To put assembler into TCM just use:: 99 100 .section ".tcm.text" or .section ".tcm.data" 101 102respectively. 103 104Example 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 } 162