sys/bhp_msg: add IPC based implementation of Bottom Half Processor

[jia200x]

Contribution description

This PR adds an IPC version of the bhp module. It allows device independent IRQ offloading using message queues.
A bhp_msg_t descriptor holds the PID of a thread that performs Bottom Half processing on reception of a message with a dedicated type. Similar to #18435, this mechanism can be adapted to existing init functions of drivers to hook in IRQ offloading.

Testing procedure

tests/unittests should pass.

Issues/PRs references

#18435
Required for adding LWIP support for radios that require IRQ offloading

[kaspar030]

This is a bad idea. Messages sent from ISR silently vanish if the receiver is not waiting (in msg_receive()), or if there's a message queue and that is full. Both will happen once there's load, so it's difficult to track down.

The effect would be easy to show with a unit test that calls bhp_msg_isr_cb() twice from an actual ISR. Without a queue, the second message will be lost. Or with a queue of size N, call bhp_msg_isr_cb` N+1 times.

I'm beginning to think we should deprecate msg_send() from ISRs and only allow msg_try_send(), which msg_send() will use anyways when called from ISR, eating the result code.

[jia200x]

As discussed in many old PRs, I was never a fan of ISR offloading using IPC and I 100% agree with what you describe. I plan to migrate all the remaining IPC based IRQ Offloading modules to Events or Thread Flags at some point.
I only introduce this module to give support for the existing modules (concretely LWIP), since they already use IPC.
IMO it shouldn't make the situation worse but at least it should speed up the Radio HAL migration.

[benpicco]

OK I see, this is needed because LWIP uses msg already for netdev events and this will integrate with it.

[benpicco]

Code looks good, but there should be some explanation as to when bhp_event and when bhp_msg should be used.

CI has some comments too.

[jia200x]

done!

[benpicco]

Static tests are still failing

[benpicco]

Please squash!

[jia200x]

squashed!