[PATCH 10/18] arm64: Introduce FIQ support

[Arnd Bergmann]

On Sat, Feb 6, 2021 at 4:37 PM Marc Zyngier <maz at kernel.org> wrote:
> On Thu, 04 Feb 2021 20:39:43 +0000, Hector Martin <marcan at marcan.st> wrote:

> > diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
> > index bf125c591116..6acfc372dc76 100644
> > --- a/arch/arm64/include/asm/assembler.h
> > +++ b/arch/arm64/include/asm/assembler.h
> > @@ -42,7 +42,11 @@
> >
> >       /* IRQ is the lowest priority flag, unconditionally unmask the rest. */
> >       .macro enable_da_f
> > +alternative_if ARM64_NEEDS_FIQ
> > +     msr     daifclr, #(8 | 4)
> > +alternative_else
> >       msr     daifclr, #(8 | 4 | 1)
> > +alternative_endif
>
> See my digression in patch 8. I really wonder what the benefit is to
> treat FIQ independently of IRQ, and we might as well generalise
> this. We could always panic on getting a FIQ on platforms that don't
> expect one.
>
> It'd be good to rope in the other interested parties (Mark for the
> early entry code, James for RAS and SError handling).

There might be another slightly hacky but less intrusive option
that could be done (almost) purely within the aic code:

* In the fiq handler code, check if normal interrupts were enabled
  when the fiq hit. Normally they are enabled, so just proceed to
  handle the timer and ipi directly

* if irq was disabled, defer the handling by doing a self-ipi
  through the aic's ipi method, and handle it from there
  when dealing with the next interrupt once interrupts get
  enabled.

This would be similar to the soft-disable feature on powerpc, which
never actually turns off interrupts from regular kernel code but
just checks a flag in local_irq_enable that gets set when a
hardirq happened.

          Arnd