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dc7a12bd | 1 | ================================================== |
bc581770 | 2 | ARM TCM (Tightly-Coupled Memory) handling in Linux |
dc7a12bd MCC |
3 | ================================================== |
4 | ||
bc581770 LW |
5 | Written by Linus Walleij <linus.walleij@stericsson.com> |
6 | ||
2d5dfb59 | 7 | Some ARM SoCs have a so-called TCM (Tightly-Coupled Memory). |
bc581770 LW |
8 | This is usually just a few (4-64) KiB of RAM inside the ARM |
9 | processor. | |
10 | ||
2d5dfb59 | 11 | Due to being embedded inside the CPU, the TCM has a |
bc581770 LW |
12 | Harvard-architecture, so there is an ITCM (instruction TCM) |
13 | and a DTCM (data TCM). The DTCM can not contain any | |
14 | instructions, but the ITCM can actually contain data. | |
15 | The size of DTCM or ITCM is minimum 4KiB so the typical | |
16 | minimum configuration is 4KiB ITCM and 4KiB DTCM. | |
17 | ||
2d5dfb59 | 18 | ARM CPUs have special registers to read out status, physical |
bc581770 LW |
19 | location and size of TCM memories. arch/arm/include/asm/cputype.h |
20 | defines a CPUID_TCM register that you can read out from the | |
21 | system control coprocessor. Documentation from ARM can be found | |
22 | at http://infocenter.arm.com, search for "TCM Status Register" | |
23 | to see documents for all CPUs. Reading this register you can | |
1dbd30e9 LW |
24 | determine if ITCM (bits 1-0) and/or DTCM (bit 17-16) is present |
25 | in the machine. | |
bc581770 LW |
26 | |
27 | There is further a TCM region register (search for "TCM Region | |
28 | Registers" at the ARM site) that can report and modify the location | |
29 | size of TCM memories at runtime. This is used to read out and modify | |
30 | TCM location and size. Notice that this is not a MMU table: you | |
31 | actually move the physical location of the TCM around. At the | |
32 | place you put it, it will mask any underlying RAM from the | |
33 | CPU so it is usually wise not to overlap any physical RAM with | |
610ea6c6 | 34 | the TCM. |
bc581770 | 35 | |
610ea6c6 LW |
36 | The TCM memory can then be remapped to another address again using |
37 | the MMU, but notice that the TCM if often used in situations where | |
38 | the MMU is turned off. To avoid confusion the current Linux | |
39 | implementation will map the TCM 1 to 1 from physical to virtual | |
1dbd30e9 LW |
40 | memory in the location specified by the kernel. Currently Linux |
41 | will map ITCM to 0xfffe0000 and on, and DTCM to 0xfffe8000 and | |
42 | on, supporting a maximum of 32KiB of ITCM and 32KiB of DTCM. | |
43 | ||
44 | Newer versions of the region registers also support dividing these | |
45 | TCMs in two separate banks, so for example an 8KiB ITCM is divided | |
46 | into two 4KiB banks with its own control registers. The idea is to | |
47 | be able to lock and hide one of the banks for use by the secure | |
48 | world (TrustZone). | |
bc581770 LW |
49 | |
50 | TCM 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 | ||
63 | There is an interface for using TCM on the ARM architecture | |
64 | in <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 | ||
1dbd30e9 LW |
78 | A machine that has TCM memory shall select HAVE_TCM from |
79 | arch/arm/Kconfig for itself. Code that needs to use TCM shall | |
80 | #include <asm/tcm.h> | |
bc581770 LW |
81 | |
82 | Functions to go into itcm can be tagged like this: | |
83 | int __tcmfunc foo(int bar); | |
84 | ||
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85 | Since these are marked to become long_calls and you may want |
86 | to have functions called locally inside the TCM without | |
87 | wasting space, there is also the __tcmlocalfunc prefix that | |
88 | will make the call relative. | |
89 | ||
dc7a12bd MCC |
90 | Variables to go into dtcm can be tagged like this:: |
91 | ||
92 | int __tcmdata foo; | |
93 | ||
94 | Constants can be tagged like this:: | |
bc581770 | 95 | |
dc7a12bd MCC |
96 | int __tcmconst foo; |
97 | ||
98 | To put assembler into TCM just use:: | |
99 | ||
100 | .section ".tcm.text" or .section ".tcm.data" | |
bc581770 | 101 | |
bc581770 LW |
102 | respectively. |
103 | ||
dc7a12bd | 104 | Example code:: |
bc581770 | 105 | |
dc7a12bd | 106 | #include <asm/tcm.h> |
bc581770 | 107 | |
dc7a12bd MCC |
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; | |
bc581770 | 114 | |
dc7a12bd MCC |
115 | static void __tcmlocalfunc tcm_to_tcm(void) |
116 | { | |
bc581770 LW |
117 | int i; |
118 | for (i = 0; i < 100; i++) | |
119 | tcmvar ++; | |
dc7a12bd | 120 | } |
bc581770 | 121 | |
dc7a12bd MCC |
122 | static void __tcmfunc hello_tcm(void) |
123 | { | |
bc581770 LW |
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(); | |
dc7a12bd | 130 | } |
bc581770 | 131 | |
dc7a12bd MCC |
132 | static void __init test_tcm(void) |
133 | { | |
bc581770 LW |
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 | } | |
dc7a12bd | 161 | } |