[riot-notifications] [RIOT-OS/RIOT] fe310: Use ecall instruction for thread yield (#15736)

Koen Zandberg notifications at github.com
Mon Jan 11 11:07:40 CET 2021

### Contribution description

This switches the thread yield on the ~~fe310~~ RISC-V CPU to use the ECALL instruction to trap the core. Currently the software interrupt is used for this, but this is supposed to be used for inter-core interrupts according to the manual.

I've kept the option open on the interface to add a number and some context to the ecall and have this passed to the trap handler. This still has to be hooked up to the trap handler itself in the future.

### Testing procedure

This is only tested on Qemu as I currently don't have hardware to test this.

Thread switching on an fe310-based board, e.g. `tests/bench_mutex_pingpong`

I'm also curious about actual numbers, this improves performance for me on Qemu, but no clue if that's also the case on real hardware.

### Issues/PRs references

Alternative to #15277 
You can view, comment on, or merge this pull request online at:


-- Commit Summary --

  * fe310: Use ecall instruction for thread yield

-- File Changes --

    M cpu/fe310/irq_arch.c (26)
    M cpu/fe310/thread_arch.c (20)

-- Patch Links --


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