Electronic – Debugging Cortex-M7 with data cache

cachedebugginggdbopenocdstm32h

I'm using gdb, openocd and stlink to debug an application running on STM32H7. When data cache (DCache) is enabled, debugger does not show correct values. For example if I run this:

int foo;
int main()
{
    foo = 1234;
    while(1);
}

debugger shows foo as 0. Apparently it is reading the values really in RAM, instead of the values in data cache like I would expect it to.

Best Answer

The Cortex-M7 processor has internal flag that identifies debugger accesses as either cacheable, or non-cacheable. Non-cacheable is the default, and it causes all requests to bypass the data cache.

Openocd tries to set the flag in stm32h7x.cfg, but that only works for low-level access debuggers. ST-Link is a high-level debugger (you can check with monitor using_hla in gdb). That means it should handle setting that flag automatically.

Turns out that the solution is simple: upgrade ST-Link firmware. I'm not sure what firmware version introduces data cache support, but V2J37S0 worked for me.

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