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authorLinus Walleij <linus.walleij@stericsson.com>2009-09-15 12:30:37 -0400
committerRussell King <rmk+kernel@arm.linux.org.uk>2009-09-15 17:11:05 -0400
commitbc581770cfdd8c17ea17d324dc05e2f9c599e7ca (patch)
treec6d2c2eb82d8a6c6851de0ecc7d8c0e63e026266 /Documentation/arm
parent18240904960a39e582ced8ba8ececb10b8c22dd3 (diff)
ARM: 5580/2: ARM TCM (Tightly-Coupled Memory) support v3
This adds the TCM interface to Linux, when active, it will detect and report TCM memories and sizes early in boot if present, introduce generic TCM memory handling, provide a generic TCM memory pool and select TCM memory for the U300 platform. See the Documentation/arm/tcm.txt for documentation. Signed-off-by: Linus Walleij <linus.walleij@stericsson.com> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
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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. The TCM memory exists totally outside the MMU and will
33override any MMU mappings.
34
35Code executing inside the ITCM does not "see" any MMU mappings
36and e.g. register accesses must be made to physical addresses.
37
38TCM is used for a few things:
39
40- FIQ and other interrupt handlers that need deterministic
41 timing and cannot wait for cache misses.
42
43- Idle loops where all external RAM is set to self-refresh
44 retention mode, so only on-chip RAM is accessible by
45 the CPU and then we hang inside ITCM waiting for an
46 interrupt.
47
48- Other operations which implies shutting off or reconfiguring
49 the external RAM controller.
50
51There is an interface for using TCM on the ARM architecture
52in <asm/tcm.h>. Using this interface it is possible to:
53
54- Define the physical address and size of ITCM and DTCM.
55
56- Tag functions to be compiled into ITCM.
57
58- Tag data and constants to be allocated to DTCM and ITCM.
59
60- Have the remaining TCM RAM added to a special
61 allocation pool with gen_pool_create() and gen_pool_add()
62 and provice tcm_alloc() and tcm_free() for this
63 memory. Such a heap is great for things like saving
64 device state when shutting off device power domains.
65
66A machine that has TCM memory shall select HAVE_TCM in
67arch/arm/Kconfig for itself, and then the
68rest of the functionality will depend on the physical
69location and size of ITCM and DTCM to be defined in
70mach/memory.h for the machine. Code that needs to use
71TCM shall #include <asm/tcm.h> If the TCM is not located
72at the place given in memory.h it will be moved using
73the TCM Region registers.
74
75Functions to go into itcm can be tagged like this:
76int __tcmfunc foo(int bar);
77
78Variables to go into dtcm can be tagged like this:
79int __tcmdata foo;
80
81Constants can be tagged like this:
82int __tcmconst foo;
83
84To put assembler into TCM just use
85.section ".tcm.text" or .section ".tcm.data"
86respectively.
87
88Example code:
89
90#include <asm/tcm.h>
91
92/* Uninitialized data */
93static u32 __tcmdata tcmvar;
94/* Initialized data */
95static u32 __tcmdata tcmassigned = 0x2BADBABEU;
96/* Constant */
97static const u32 __tcmconst tcmconst = 0xCAFEBABEU;
98
99static void __tcmlocalfunc tcm_to_tcm(void)
100{
101 int i;
102 for (i = 0; i < 100; i++)
103 tcmvar ++;
104}
105
106static void __tcmfunc hello_tcm(void)
107{
108 /* Some abstract code that runs in ITCM */
109 int i;
110 for (i = 0; i < 100; i++) {
111 tcmvar ++;
112 }
113 tcm_to_tcm();
114}
115
116static void __init test_tcm(void)
117{
118 u32 *tcmem;
119 int i;
120
121 hello_tcm();
122 printk("Hello TCM executed from ITCM RAM\n");
123
124 printk("TCM variable from testrun: %u @ %p\n", tcmvar, &tcmvar);
125 tcmvar = 0xDEADBEEFU;
126 printk("TCM variable: 0x%x @ %p\n", tcmvar, &tcmvar);
127
128 printk("TCM assigned variable: 0x%x @ %p\n", tcmassigned, &tcmassigned);
129
130 printk("TCM constant: 0x%x @ %p\n", tcmconst, &tcmconst);
131
132 /* Allocate some TCM memory from the pool */
133 tcmem = tcm_alloc(20);
134 if (tcmem) {
135 printk("TCM Allocated 20 bytes of TCM @ %p\n", tcmem);
136 tcmem[0] = 0xDEADBEEFU;
137 tcmem[1] = 0x2BADBABEU;
138 tcmem[2] = 0xCAFEBABEU;
139 tcmem[3] = 0xDEADBEEFU;
140 tcmem[4] = 0x2BADBABEU;
141 for (i = 0; i < 5; i++)
142 printk("TCM tcmem[%d] = %08x\n", i, tcmem[i]);
143 tcm_free(tcmem, 20);
144 }
145}