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+ARM TCM (Tightly-Coupled Memory) handling in Linux
+----
+Written by Linus Walleij <linus.walleij@stericsson.com>
+
+Some ARM SoC:s have a so-called TCM (Tightly-Coupled Memory).
+This is usually just a few (4-64) KiB of RAM inside the ARM
+processor.
+
+Due to being embedded inside the CPU The TCM has a
+Harvard-architecture, so there is an ITCM (instruction TCM)
+and a DTCM (data TCM). The DTCM can not contain any
+instructions, but the ITCM can actually contain data.
+The size of DTCM or ITCM is minimum 4KiB so the typical
+minimum configuration is 4KiB ITCM and 4KiB DTCM.
+
+ARM CPU:s have special registers to read out status, physical
+location and size of TCM memories. arch/arm/include/asm/cputype.h
+defines a CPUID_TCM register that you can read out from the
+system control coprocessor. Documentation from ARM can be found
+at http://infocenter.arm.com, search for "TCM Status Register"
+to see documents for all CPUs. Reading this register you can
+determine if ITCM (bit 0) and/or DTCM (bit 16) is present in the
+machine.
+
+There is further a TCM region register (search for "TCM Region
+Registers" at the ARM site) that can report and modify the location
+size of TCM memories at runtime. This is used to read out and modify
+TCM location and size. Notice that this is not a MMU table: you
+actually move the physical location of the TCM around. At the
+place you put it, it will mask any underlying RAM from the
+CPU so it is usually wise not to overlap any physical RAM with
+the TCM. The TCM memory exists totally outside the MMU and will
+override any MMU mappings.
+
+Code executing inside the ITCM does not "see" any MMU mappings
+and e.g. register accesses must be made to physical addresses.
+
+TCM is used for a few things:
+
+- FIQ and other interrupt handlers that need deterministic
+  timing and cannot wait for cache misses.
+
+- Idle loops where all external RAM is set to self-refresh
+  retention mode, so only on-chip RAM is accessible by
+  the CPU and then we hang inside ITCM waiting for an
+  interrupt.
+
+- Other operations which implies shutting off or reconfiguring
+  the external RAM controller.
+
+There is an interface for using TCM on the ARM architecture
+in <asm/tcm.h>. Using this interface it is possible to:
+
+- Define the physical address and size of ITCM and DTCM.
+
+- Tag functions to be compiled into ITCM.
+
+- Tag data and constants to be allocated to DTCM and ITCM.
+
+- Have the remaining TCM RAM added to a special
+  allocation pool with gen_pool_create() and gen_pool_add()
+  and provice tcm_alloc() and tcm_free() for this
+  memory. Such a heap is great for things like saving
+  device state when shutting off device power domains.
+
+A machine that has TCM memory shall select HAVE_TCM in
+arch/arm/Kconfig for itself, and then the
+rest of the functionality will depend on the physical
+location and size of ITCM and DTCM to be defined in
+mach/memory.h for the machine. Code that needs to use
+TCM shall #include <asm/tcm.h> If the TCM is not located
+at the place given in memory.h it will be moved using
+the TCM Region registers.
+
+Functions to go into itcm can be tagged like this:
+int __tcmfunc foo(int bar);
+
+Variables to go into dtcm can be tagged like this:
+int __tcmdata foo;
+
+Constants can be tagged like this:
+int __tcmconst foo;
+
+To put assembler into TCM just use
+.section ".tcm.text" or .section ".tcm.data"
+respectively.
+
+Example code:
+
+#include <asm/tcm.h>
+
+/* Uninitialized data */
+static u32 __tcmdata tcmvar;
+/* Initialized data */
+static u32 __tcmdata tcmassigned = 0x2BADBABEU;
+/* Constant */
+static const u32 __tcmconst tcmconst = 0xCAFEBABEU;
+
+static void __tcmlocalfunc tcm_to_tcm(void)
+{
+        int i;
+        for (i = 0; i < 100; i++)
+                tcmvar ++;
+}
+
+static void __tcmfunc hello_tcm(void)
+{
+        /* Some abstract code that runs in ITCM */
+        int i;
+        for (i = 0; i < 100; i++) {
+                tcmvar ++;
+        }
+        tcm_to_tcm();
+}
+
+static void __init test_tcm(void)
+{
+        u32 *tcmem;
+        int i;
+
+        hello_tcm();
+        printk("Hello TCM executed from ITCM RAM\n");
+
+        printk("TCM variable from testrun: %u @ %p\n", tcmvar, &tcmvar);
+        tcmvar = 0xDEADBEEFU;
+        printk("TCM variable: 0x%x @ %p\n", tcmvar, &tcmvar);
+
+        printk("TCM assigned variable: 0x%x @ %p\n", tcmassigned, &tcmassigned);
+
+        printk("TCM constant: 0x%x @ %p\n", tcmconst, &tcmconst);
+
+        /* Allocate some TCM memory from the pool */
+        tcmem = tcm_alloc(20);
+        if (tcmem) {
+                printk("TCM Allocated 20 bytes of TCM @ %p\n", tcmem);
+                tcmem[0] = 0xDEADBEEFU;
+                tcmem[1] = 0x2BADBABEU;
+                tcmem[2] = 0xCAFEBABEU;
+                tcmem[3] = 0xDEADBEEFU;
+                tcmem[4] = 0x2BADBABEU;
+                for (i = 0; i < 5; i++)
+                        printk("TCM tcmem[%d] = %08x\n", i, tcmem[i]);
+                tcm_free(tcmem, 20);
+        }
+}