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 ARMprocessor.

Due to being embedded inside the CPU, the TCM has aHarvard-architecture, so there is an ITCM (instruction TCM)and a DTCM (data TCM). The DTCM can not contain anyinstructions, but the ITCM can actually contain data.The size of DTCM or ITCM is minimum 4KiB so the typicalminimum configuration is 4KiB ITCM and 4KiB DTCM.

ARM CPUs have special registers to read out status, physicallocation and size of TCM memories. arch/arm/include/asm/cputype.hdefines a CPUID_TCM register that you can read out from thesystem control coprocessor. Documentation from ARM can be foundathttp://infocenter.arm.com, search for “TCM Status Register”to see documents for all CPUs. Reading this register you candetermine if ITCM (bits 1-0) and/or DTCM (bit 17-16) is presentin the machine.

There is further a TCM region register (search for “TCM RegionRegisters” at the ARM site) that can report and modify the locationsize of TCM memories at runtime. This is used to read out and modifyTCM location and size. Notice that this is not a MMU table: youactually move the physical location of the TCM around. At theplace you put it, it will mask any underlying RAM from theCPU so it is usually wise not to overlap any physical RAM withthe TCM.

The TCM memory can then be remapped to another address again usingthe MMU, but notice that the TCM if often used in situations wherethe MMU is turned off. To avoid confusion the current Linuximplementation will map the TCM 1 to 1 from physical to virtualmemory in the location specified by the kernel. Currently Linuxwill map ITCM to 0xfffe0000 and on, and DTCM to 0xfffe8000 andon, supporting a maximum of 32KiB of ITCM and 32KiB of DTCM.

Newer versions of the region registers also support dividing theseTCMs in two separate banks, so for example an 8KiB ITCM is dividedinto two 4KiB banks with its own control registers. The idea is tobe able to lock and hide one of the banks for use by the secureworld (TrustZone).

TCM is used for a few things:

  • FIQ and other interrupt handlers that need deterministictiming and cannot wait for cache misses.
  • Idle loops where all external RAM is set to self-refreshretention mode, so only on-chip RAM is accessible bythe CPU and then we hang inside ITCM waiting for aninterrupt.
  • Other operations which implies shutting off or reconfiguringthe external RAM controller.

There is an interface for using TCM on the ARM architecturein <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 specialallocation pool withgen_pool_create() andgen_pool_add()and provice tcm_alloc() and tcm_free() for thismemory. Such a heap is great for things like savingdevice state when shutting off device power domains.

A machine that has TCM memory shall select HAVE_TCM fromarch/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 wantto have functions called locally inside the TCM withoutwasting space, there is also the __tcmlocalfunc prefix thatwill 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);      }}