Re: [PATCH v6 3/6] irqchip: RISC-V per-HART local interrupt controller driver

[Marc Zyngier]

On 2020-05-31 06:36, Anup Patel wrote:
> On Sat, May 30, 2020 at 5:31 PM Marc Zyngier <maz@xxxxxxxxxx> wrote:

[...]

> > >       plic_set_threshold(handler, PLIC_DISABLE_THRESHOLD);
> >
> > Why do you need to both disable the interrupt *and* change the 
> > priority
> > threshold? It seems to be that one of them should be enough, but my
> > kno9wledge of the PLIC is limited. In any case, this would deserve a
> > comment.
>
> Okay, I will test and remove "disable the interrupt" part from 
> plic_dying_cpu().

Be careful, as interrupt enabling/disabling is refcounted in order
to allow nesting. If you only enable on CPU_ON and not disable
on CPU_OFF, you will end-up with a depth that only increases,
up to the point where you hit the roof (it will take a while though).

I would keep the enable/disable as is, and drop the priority
setting from the CPU_OFF path.

> > >       return 0;
> > > @@ -260,7 +266,11 @@ static int plic_starting_cpu(unsigned int cpu)
> > >  {
> > >       struct plic_handler *handler = this_cpu_ptr(&plic_handlers);
> > >
> > > -     csr_set(CSR_IE, IE_EIE);
> > > +     if (plic_parent_irq)
> > > +             enable_percpu_irq(plic_parent_irq,
> > > +                               irq_get_trigger_type(plic_parent_irq));
> > > +     else
> > > +             pr_warn("cpu%d: parent irq not available\n");
> >
> > What does it mean to carry on if the interrupt cannot be signaled?
> > Shouldn't you error out instead, and leave the CPU dead?
>
> The CPU is not dead if we cannot enable RISC-V INTC external
> interrupt because the Timer and IPIs interrupts are always through
> RISC-V INTC. The PLIC external interrupt not present for a CPU
> only means that that CPU cannot receive peripherial interrupts.
>
> On a sane RISC-V system, if PLIC is present then all CPUs should
> be able to get RISC-V INTC external interrupt. Base on this rationale,
> I have put a warning for plic_parent_irq == 0.

Fair enough.

        M.
--
Jazz is not dead. It just smells funny...