Trying to debug with MCUBOOT enabled fails to boot.

[MatjazK]

Hello,
I am looking for a way to debug firmware that includes MCUBOOT. There seem to be quite some questions in regards to same problem with no real resolution.
I am using various versions of JLink, it seems that the problem occurs when flash breakpoint functionality does its magic.

From my understanding what happens:
- I flash firmware to target MCU (in this case it is nRF52840)
- Device boots normally, MCUBOOT does its magic and all the checks seem fine.
- I start debug session, device reboots into MCUBOOT and sets breakpoint to main as entry point to run to - I have no other breakpoints enabled!
- Because debugger is using flash breakpoints, it will replace some instructions in flash with a breakpoint instruction.
- Because debugger modified flash contents, application firmware image integrity is no longer the same as MCUBOOT expects.
- MCUBOOT therefore fails to boot with no bootable image found/FIH_PANIC.

I already tried all the MCUBOOT related kconfigs, that are supposed to disable checks on image:
- enable BOOT_SIGNATURE_TYPE_NONE
- disable BOOT_VALIDATE_SLOT0
- BOOT_VALIDATE_SLOT0_ONCE doesn't solve the problem either
- enable BOOT_FIH_PROFILE_OFF

How can I make sure that I will be able to debug firmware, despite using mcuboot? I don't think disabling it just for the sake of being able to debug the code is really suitable solution to the issue. I noticed that this issue does seem to depend on debugging hardware being used and its support for flash breakpoints and number of HW breakpoints being supported. But there also seems no exact clear requirement.

[Torsten Robitzki]

Basically, the Cortex M4 core should have enough hardware breakpoints for basic debugging. I would try to find a way, to disable Seggers software break points, to see if this makes a differences.

[MatjazK]

Any idea how to achieve that? I have tried various things in effort to disable those with no luck.
Another interesting finding is, that using "debug" command breaks bootloader, using "attach" however - works fine. Even when resetting MCU after attach bootloader works fine

[Torsten Robitzki]

According to the documentation, there is a GDB monitor command to disable flash breakpoints: https://wiki.segger.com/J-Link_GDB_Server#flash_breakpoints

[MatjazK]

Looking a bit further into this I found I need two things to make sure this works:
- disable flash breakpoint
- load mcuboots .elf file during debug launch

for cortex debugs launch.json configuration on VSCode that was resolved by adding following lines to debug launch:

      "loadFiles": [
        "/workspaces/project/app/build/mcuboot/zephyr/zephyr.elf"
      ],
      "preLaunchCommands": ["monitor flash breakpoints = 0"]