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bc581770 LW |
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 | |
1dbd30e9 LW |
22 | determine if ITCM (bits 1-0) and/or DTCM (bit 17-16) is present |
23 | in the machine. | |
bc581770 LW |
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 | |
610ea6c6 | 32 | the TCM. |
bc581770 | 33 | |
610ea6c6 LW |
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 | |
1dbd30e9 LW |
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). | |
bc581770 LW |
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 | ||
1dbd30e9 LW |
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> | |
bc581770 LW |
79 | |
80 | Functions to go into itcm can be tagged like this: | |
81 | int __tcmfunc foo(int bar); | |
82 | ||
1dbd30e9 LW |
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 | ||
bc581770 LW |
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 | } |