Debugging on Nucleo H755ZI-Q doesn't work properly

[mthiede-acn2]

Describe the bug

Debugging using west debug does not work properly.
For example, single stepping is not possible.
si will not run the next instruction.
n and s also don't work as well as a c after hitting a breakpoint.

I updated the firmware of the STLink to the latest version.
I tried halting the M4 core using the BOOT_CM4 option without success.
I get the same behavior with Zephyr v4.1.0

To Reproduce

1. west build -b nucleo_h755zi_q/stm32h755xx/m7 -p always samples/basic/blinky
2. west debug
3. b main
4. c
5. n

Expected behavior
Debugger single steps to next instruction

Impact
Debugging hardly possible. It is possible to set and break on a first breakpoint though (like on 'main' above)

Environment (please complete the following information):

• OS: WSL/Ubuntu
• Toolchain: Zephyr SDK 0.17.0
• Zephyr: v4.0.0 (release tag), same with v4.1.0
• Open On-Chip Debugger 0.11.0+dev-00728-gb6f95a16c-dirty (from SDK 0.17.0)
• GNU gdb (Zephyr SDK 0.17.0) 12.1

[erwango]

I don't have this board, so I can't test, but I had a quick test on stm32h747i_disco/stm32h747xx/m7 and debug works reliabily.
Though, openocd config file for Nucleo H755ZI-Q isn't aligned on stm32h747i_disco, so it should be aligned.
Would you be able to import the configuration and have a try ?

About M4 debug, IIRC there were some attempts to have it functional with gdb, but from what I could test it doesn't work. The related note in the board documentation still stands:

Debugging a Zephyr application on Cortex M4 side with west is currently not
available. As a workaround, `STM32CubeIDE`_ can be used.

[lromeraj]

Did you build the application without size optimizations (default mode)? Try enabling CONFIG_DEBUG=y and ensuring CONFIG_DEBUG_OPTIMIZATIONS=y, then reflash the firmware and check if it improves the debugging process.

[mthiede-acn2]

@erwango I modified zephyr/boards/st/nucleo_h755zi_q/support/openocd.cfg to contain line

source [find board/stm32h745i-disco.cfg]

instead of

source [find board/st_nucleo_h745zi.cfg]

and now single stepping and continue work.

@lromeraj thanks for the hint, this improves debug experience once debugging is working at all

[erwango]

@mthiede-acn2 Thanks for the feedback. Would you have the possibility to push this change so everyone can benefit ?

[mthiede-acn2]

@erwango I've created #87541

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