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2 ARM TCM (Tightly-Coupled Memory) handling in Linux
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4
5 Written by Linus Walleij <linus.walleij@stericsson.com>
6
7 Some ARM SoCs have a so-called TCM (Tightly-Coupled Memory).
8 This is usually just a few (4-64) KiB of RAM inside the ARM
9 processor.
10
11 Due to being embedded inside the CPU, the TCM has a
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
18 ARM CPUs have special registers to read out status, physical
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
24 determine if ITCM (bits 1-0) and/or DTCM (bit 17-16) is present
25 in the machine.
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
34 the TCM.
35
36 The TCM memory can then be remapped to another address again using
37 the MMU, but notice that the TCM is 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
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).
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 provide 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
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>
81
82 Functions to go into itcm can be tagged like this:
83 int __tcmfunc foo(int bar);
84
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
90 Variables to go into dtcm can be tagged like this::
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 = 0x2BADBABEU;
112 /* Constant */
113 static const u32 __tcmconst tcmconst = 0xCAFEBABEU;
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 ITCM */
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 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 }
161 }
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