This is a minimalistic SBI implementation for NuttX.
Provides a single service for now:
- Access to machine timer
Provides a start trampoline to start NuttX in S-mode:
- Exceptions / faults are delegated to S-mode.
- External interrupts are delegated to S-mode.
Machine mode timer is used as follows:
- The timer compare match register reload happens in M-mode, via
call gate "riscv_sbi_set_timer"
- The compare match event is dispatched to S-mode ISR, which will
notify the kernel to advance time
- Clearing the STIP interrupt does not work from S-mode,
so the call gate does this from M-mode
The only supported (tested) target for now is MPFS.
