Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.
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 }