1 ==============================================
2 ARM TCM (Tightly-Coupled Memory) handling in L
3 ==============================================
4
5 Written by Linus Walleij <linus.walleij@sterics
6
7 Some ARM SoCs have a so-called TCM (Tightly-Co
8 This is usually just a few (4-64) KiB of RAM i
9 processor.
10
11 Due to being embedded inside the CPU, the TCM
12 Harvard-architecture, so there is an ITCM (ins
13 and a DTCM (data TCM). The DTCM can not contai
14 instructions, but the ITCM can actually contai
15 The size of DTCM or ITCM is minimum 4KiB so th
16 minimum configuration is 4KiB ITCM and 4KiB DT
17
18 ARM CPUs have special registers to read out st
19 location and size of TCM memories. arch/arm/in
20 defines a CPUID_TCM register that you can read
21 system control coprocessor. Documentation from
22 at http://infocenter.arm.com, search for "TCM
23 to see documents for all CPUs. Reading this re
24 determine if ITCM (bits 1-0) and/or DTCM (bit
25 in the machine.
26
27 There is further a TCM region register (search
28 Registers" at the ARM site) that can report an
29 size of TCM memories at runtime. This is used
30 TCM location and size. Notice that this is not
31 actually move the physical location of the TCM
32 place you put it, it will mask any underlying
33 CPU so it is usually wise not to overlap any p
34 the TCM.
35
36 The TCM memory can then be remapped to another
37 the MMU, but notice that the TCM is often used
38 the MMU is turned off. To avoid confusion the
39 implementation will map the TCM 1 to 1 from ph
40 memory in the location specified by the kernel
41 will map ITCM to 0xfffe0000 and on, and DTCM t
42 on, supporting a maximum of 32KiB of ITCM and
43
44 Newer versions of the region registers also su
45 TCMs in two separate banks, so for example an
46 into two 4KiB banks with its own control regis
47 be able to lock and hide one of the banks for
48 world (TrustZone).
49
50 TCM is used for a few things:
51
52 - FIQ and other interrupt handlers that need d
53 timing and cannot wait for cache misses.
54
55 - Idle loops where all external RAM is set to
56 retention mode, so only on-chip RAM is acces
57 the CPU and then we hang inside ITCM waiting
58 interrupt.
59
60 - Other operations which implies shutting off
61 the external RAM controller.
62
63 There is an interface for using TCM on the ARM
64 in <asm/tcm.h>. Using this interface it is pos
65
66 - Define the physical address and size of ITCM
67
68 - Tag functions to be compiled into ITCM.
69
70 - Tag data and constants to be allocated to DT
71
72 - Have the remaining TCM RAM added to a specia
73 allocation pool with gen_pool_create() and g
74 and provide tcm_alloc() and tcm_free() for t
75 memory. Such a heap is great for things like
76 device state when shutting off device power
77
78 A machine that has TCM memory shall select HAV
79 arch/arm/Kconfig for itself. Code that needs t
80 #include <asm/tcm.h>
81
82 Functions to go into itcm can be tagged like t
83 int __tcmfunc foo(int bar);
84
85 Since these are marked to become long_calls an
86 to have functions called locally inside the TC
87 wasting space, there is also the __tcmlocalfun
88 will make the call relative.
89
90 Variables to go into dtcm can be tagged like t
91
92 int __tcmdata foo;
93
94 Constants can be tagged like this::
95
96 int __tcmconst foo;
97
98 To put assembler into TCM just use::
99
100 .section ".tcm.text" or .section ".tcm.data"
101
102 respectively.
103
104 Example code::
105
106 #include <asm/tcm.h>
107
108 /* Uninitialized data */
109 static u32 __tcmdata tcmvar;
110 /* Initialized data */
111 static u32 __tcmdata tcmassigned = 0x2BADBAB
112 /* Constant */
113 static const u32 __tcmconst tcmconst = 0xCAF
114
115 static void __tcmlocalfunc tcm_to_tcm(void)
116 {
117 int i;
118 for (i = 0; i < 100; i++)
119 tcmvar ++;
120 }
121
122 static void __tcmfunc hello_tcm(void)
123 {
124 /* Some abstract code that runs in ITC
125 int i;
126 for (i = 0; i < 100; i++) {
127 tcmvar ++;
128 }
129 tcm_to_tcm();
130 }
131
132 static void __init test_tcm(void)
133 {
134 u32 *tcmem;
135 int i;
136
137 hello_tcm();
138 printk("Hello TCM executed from ITCM R
139
140 printk("TCM variable from testrun: %u
141 tcmvar = 0xDEADBEEFU;
142 printk("TCM variable: 0x%x @ %p\n", tc
143
144 printk("TCM assigned variable: 0x%x @
145
146 printk("TCM constant: 0x%x @ %p\n", tc
147
148 /* Allocate some TCM memory from the p
149 tcmem = tcm_alloc(20);
150 if (tcmem) {
151 printk("TCM Allocated 20 bytes
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]
159 tcm_free(tcmem, 20);
160 }
161 }
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.