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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 (bit 0) and/or DTCM (bit 16) is present in the |
23 | 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 machine. |
39 | |
40 | TCM is used for a few things: |
41 | |
42 | - FIQ and other interrupt handlers that need deterministic |
43 | timing and cannot wait for cache misses. |
44 | |
45 | - Idle loops where all external RAM is set to self-refresh |
46 | retention mode, so only on-chip RAM is accessible by |
47 | the CPU and then we hang inside ITCM waiting for an |
48 | interrupt. |
49 | |
50 | - Other operations which implies shutting off or reconfiguring |
51 | the external RAM controller. |
52 | |
53 | There is an interface for using TCM on the ARM architecture |
54 | in <asm/tcm.h>. Using this interface it is possible to: |
55 | |
56 | - Define the physical address and size of ITCM and DTCM. |
57 | |
58 | - Tag functions to be compiled into ITCM. |
59 | |
60 | - Tag data and constants to be allocated to DTCM and ITCM. |
61 | |
62 | - Have the remaining TCM RAM added to a special |
63 | allocation pool with gen_pool_create() and gen_pool_add() |
64 | and provice tcm_alloc() and tcm_free() for this |
65 | memory. Such a heap is great for things like saving |
66 | device state when shutting off device power domains. |
67 | |
68 | A machine that has TCM memory shall select HAVE_TCM in |
69 | arch/arm/Kconfig for itself, and then the |
70 | rest of the functionality will depend on the physical |
71 | location and size of ITCM and DTCM to be defined in |
72 | mach/memory.h for the machine. Code that needs to use |
73 | TCM shall #include <asm/tcm.h> If the TCM is not located |
74 | at the place given in memory.h it will be moved using |
75 | the TCM Region registers. |
76 | |
77 | Functions to go into itcm can be tagged like this: |
78 | int __tcmfunc foo(int bar); |
79 | |
80 | Variables to go into dtcm can be tagged like this: |
81 | int __tcmdata foo; |
82 | |
83 | Constants can be tagged like this: |
84 | int __tcmconst foo; |
85 | |
86 | To put assembler into TCM just use |
87 | .section ".tcm.text" or .section ".tcm.data" |
88 | respectively. |
89 | |
90 | Example code: |
91 | |
92 | #include <asm/tcm.h> |
93 | |
94 | /* Uninitialized data */ |
95 | static u32 __tcmdata tcmvar; |
96 | /* Initialized data */ |
97 | static u32 __tcmdata tcmassigned = 0x2BADBABEU; |
98 | /* Constant */ |
99 | static const u32 __tcmconst tcmconst = 0xCAFEBABEU; |
100 | |
101 | static void __tcmlocalfunc tcm_to_tcm(void) |
102 | { |
103 | int i; |
104 | for (i = 0; i < 100; i++) |
105 | tcmvar ++; |
106 | } |
107 | |
108 | static void __tcmfunc hello_tcm(void) |
109 | { |
110 | /* Some abstract code that runs in ITCM */ |
111 | int i; |
112 | for (i = 0; i < 100; i++) { |
113 | tcmvar ++; |
114 | } |
115 | tcm_to_tcm(); |
116 | } |
117 | |
118 | static void __init test_tcm(void) |
119 | { |
120 | u32 *tcmem; |
121 | int i; |
122 | |
123 | hello_tcm(); |
124 | printk("Hello TCM executed from ITCM RAM\n"); |
125 | |
126 | printk("TCM variable from testrun: %u @ %p\n", tcmvar, &tcmvar); |
127 | tcmvar = 0xDEADBEEFU; |
128 | printk("TCM variable: 0x%x @ %p\n", tcmvar, &tcmvar); |
129 | |
130 | printk("TCM assigned variable: 0x%x @ %p\n", tcmassigned, &tcmassigned); |
131 | |
132 | printk("TCM constant: 0x%x @ %p\n", tcmconst, &tcmconst); |
133 | |
134 | /* Allocate some TCM memory from the pool */ |
135 | tcmem = tcm_alloc(20); |
136 | if (tcmem) { |
137 | printk("TCM Allocated 20 bytes of TCM @ %p\n", tcmem); |
138 | tcmem[0] = 0xDEADBEEFU; |
139 | tcmem[1] = 0x2BADBABEU; |
140 | tcmem[2] = 0xCAFEBABEU; |
141 | tcmem[3] = 0xDEADBEEFU; |
142 | tcmem[4] = 0x2BADBABEU; |
143 | for (i = 0; i < 5; i++) |
144 | printk("TCM tcmem[%d] = %08x\n", i, tcmem[i]); |
145 | tcm_free(tcmem, 20); |
146 | } |
147 | } |
148 |
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