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1ARM TCM (Tightly-Coupled Memory) handling in Linux
2----
3Written by Linus Walleij <linus.walleij@stericsson.com>
4
5Some ARM SoC:s have a so-called TCM (Tightly-Coupled Memory).
6This is usually just a few (4-64) KiB of RAM inside the ARM
7processor.
8
9Due to being embedded inside the CPU The TCM has a
10Harvard-architecture, so there is an ITCM (instruction TCM)
11and a DTCM (data TCM). The DTCM can not contain any
12instructions, but the ITCM can actually contain data.
13The size of DTCM or ITCM is minimum 4KiB so the typical
14minimum configuration is 4KiB ITCM and 4KiB DTCM.
15
16ARM CPU:s have special registers to read out status, physical
17location and size of TCM memories. arch/arm/include/asm/cputype.h
18defines a CPUID_TCM register that you can read out from the
19system control coprocessor. Documentation from ARM can be found
20at http://infocenter.arm.com, search for "TCM Status Register"
21to see documents for all CPUs. Reading this register you can
22determine if ITCM (bit 0) and/or DTCM (bit 16) is present in the
23machine.
24
25There is further a TCM region register (search for "TCM Region
26Registers" at the ARM site) that can report and modify the location
27size of TCM memories at runtime. This is used to read out and modify
28TCM location and size. Notice that this is not a MMU table: you
29actually move the physical location of the TCM around. At the
30place you put it, it will mask any underlying RAM from the
31CPU so it is usually wise not to overlap any physical RAM with
32the TCM.
33
34The TCM memory can then be remapped to another address again using
35the MMU, but notice that the TCM if often used in situations where
36the MMU is turned off. To avoid confusion the current Linux
37implementation will map the TCM 1 to 1 from physical to virtual
38memory in the location specified by the machine.
39
40TCM 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
53There is an interface for using TCM on the ARM architecture
54in <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
68A machine that has TCM memory shall select HAVE_TCM in
69arch/arm/Kconfig for itself, and then the
70rest of the functionality will depend on the physical
71location and size of ITCM and DTCM to be defined in
72mach/memory.h for the machine. Code that needs to use
73TCM shall #include <asm/tcm.h> If the TCM is not located
74at the place given in memory.h it will be moved using
75the TCM Region registers.
76
77Functions to go into itcm can be tagged like this:
78int __tcmfunc foo(int bar);
79
80Variables to go into dtcm can be tagged like this:
81int __tcmdata foo;
82
83Constants can be tagged like this:
84int __tcmconst foo;
85
86To put assembler into TCM just use
87.section ".tcm.text" or .section ".tcm.data"
88respectively.
89
90Example code:
91
92#include <asm/tcm.h>
93
94/* Uninitialized data */
95static u32 __tcmdata tcmvar;
96/* Initialized data */
97static u32 __tcmdata tcmassigned = 0x2BADBABEU;
98/* Constant */
99static const u32 __tcmconst tcmconst = 0xCAFEBABEU;
100
101static void __tcmlocalfunc tcm_to_tcm(void)
102{
103 int i;
104 for (i = 0; i < 100; i++)
105 tcmvar ++;
106}
107
108static 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
118static 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}