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Based on kernel version 3.13. Page generated on 2014-01-20 22:00 EST.

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	}
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