Background:
Was debugging on STM32H7xx when I noticed that a variable that was passed by address to a function did not correctly display the value it was updated to. Where it is used 2 lines down the variable evaluates correctly (the code functions as expected).
Some specific details of the configuration:
- STM32H743BITx
- Data cache ENABLED
- Variable located in AXI SRAM (D3)
- Optimization level -O0 (lowest optimization level)
- Debugging over SWD interface
- Keil uVision 5 (MDK ARM Essential V5.28.0.0
- ST-Link/V2 with firmware V2.J37.S7, STMicroelectronics Windows Driver V2.1.0.0 signed: STMICROELECTRONICS (GRENOBLE 2) SAS
Investigation:
- Globally disabling cache solves the problem (but this is not an actual option)
- Locally cleaning caches by address for for specific variable immediately correctly updates variable in watch/memory windows (this implies debugger is reading directly from SRAM (AXI in my case))
- STM Document "STM32H7-System-Debug" confirms debug acts as bus master with memory access via both (1) the processor core (AHB-AP0 load/store) and the AHB bus matrix which includes the AXI SRAM (D3) among other things.
- For some reason the debugger command act directly on the AXI SRAM and not via the processor core which will actually consider the cache state.
- After further searching came across similar report with strong theory of the problem: https://electronics.stackexchange.com/questions/552477/debugging-cortex-m7-with-data-cache
- So this behavior is related to ARM CoreSight (light) AHB-AP status word register (CSW) option HPROT[4:0] - "Reset value: non-secure, non-exclusive, noncacheable, non-bufferable, data access, privileged = b00011". This must be set changed to cacheable as per this GDB implementation: stm32h7x.cfg
See: https://developer.arm.com/documentation/ddi0314/h/debug-access-port/ahb-ap/programmers-model-overview?lang=en
Solutions attempted:
- Tried to upgrade my STLink/V2 firmware to V2J37S7. This on its own did not solve the problem.
- Tried to find bugfix for STLink/V2 firmware that references the problem without success
Any help and suggestions would be appreciated. Please consider that I do not have much flexibility around IDE, debugger.
