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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 (bits 1-0) and/or DTCM (bit 17-16) is present
23 in the 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 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).
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
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>
79
80 Functions to go into itcm can be tagged like this:
81 int __tcmfunc foo(int bar);
82
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
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 }