Commit | Line | Data |
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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 | |
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 | |
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 | |
38 | memory in the location specified by the machine. | |
bc581770 LW |
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 | } |