ARM TCM (Tightly-Coupled Memory) handling in Linux — The Linux Kernel 5.18.5-gnu-1 documentation

# ARM TCM (Tightly-Coupled Memory) handling in Linux

Written by Linus Walleij <linus.walleij@stericsson.com>

Some ARM SoCs 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 CPUs 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 (bits 1-0) and/or DTCM (bit 17-16) 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 can then be remapped to another address again using the MMU, but notice that the TCM if often used in situations where the MMU is turned off. To avoid confusion the current Linux implementation will map the TCM 1 to 1 from physical to virtual memory in the location specified by the kernel. Currently Linux will map ITCM to 0xfffe0000 and on, and DTCM to 0xfffe8000 and on, supporting a maximum of 32KiB of ITCM and 32KiB of DTCM.

Newer versions of the region registers also support dividing these TCMs in two separate banks, so for example an 8KiB ITCM is divided into two 4KiB banks with its own control registers. The idea is to be able to lock and hide one of the banks for use by the secure world (TrustZone).

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 from arch/arm/Kconfig for itself. Code that needs to use TCM shall #include <asm/tcm.h>

Functions to go into itcm can be tagged like this: int __tcmfunc foo(int bar);

Since these are marked to become long_calls and you may want to have functions called locally inside the TCM without wasting space, there is also the __tcmlocalfunc prefix that will make the call relative.

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);
          }
    }