cpu/cortexm: rework bkpt instruction call on panic

[dylad]

Contribution description

Rework the way bkpt instruction is call on panic for Cortex-M MCU.
Currently, this instruction is called everytime on panic. However, when a debug session is not active (no GDB running), and if a panic occured, the bkpt instruction will either escalate the current fault to hardfault or the CPU will enter LOCKUP state (it may reset automatically depending on how vendors handle this signal internally).

To mitigate this issue, one can check the C_DEBUGEN in CoreDebug->DHCSR. If this bit is set, it means a debug session is running, thus bkpt instruction can be used safely. Otherwise, skip this instruction. The execution will then continue until pm_off() or while(1) {} is called.

However, this bit is NOT accessible by the CPU on CortexM0+ (Only accessible by GDB). So the bkpt instruction is completely skip for this one.

Testing procedure

Trigger a fault on any cortexm-based board
for instance

volatile uint32_t *pointer = (uint32_t*) 0x60000001;
uint32_t val = *pointer; 

Such code will trigger a hardfault (technically a busfault for ARMv7-M or ARMv8-M Mainline, but it escalates to hardfault as the busfault is currently not enable).
On panic, if GDB is NOT used, the execution will 'stop' on the bkpt instruction. On nRF5340DK, a reset is issued (Probably because the processor enters LOCKUP state). Nevertheless, the execution should continue up to here.

Issues/PRs references

None.

[kaspar030]

I like this idea!

[riot-ci]

Murdock results

✔️ PASSED

e784794 cpu/cortexm: uncrustify panic.c

Success	Failures	Total	Runtime
10066	0	10066	12m:33s

Artifacts

• Documentation preview

[maribu]

lgtm, thx. Please squash the nitpicks right away.

When you have the file open anyway, care to sneak in a style fix as separate commit in line 30?

-     if(ipsr) {
+     if (ipsr) {

[dylad]

@maribu I've addressed your comments and squashed.

[dylad]

Thanks!

[maribu]

Thx for the enhancement :) It will be a real quality of life improvement when the MCU no longer gets locked up :)