[PATCH V4 1/3] irqchip/sifive-plic: Add thead,c900-plic support

[Guo Ren]

On Tue, Oct 19, 2021 at 6:18 PM Marc Zyngier <maz at kernel.org> wrote:
>
> On Tue, 19 Oct 2021 10:33:49 +0100,
> Guo Ren <guoren at kernel.org> wrote:
>
> > > If you have an 'automask' behavior and yet the HW doesn't record this
> > > in a separate bit, then you need to track this by yourself in the
> > > irq_eoi() callback instead. I guess that you would skip the write to
> > > the CLAIM register in this case, though I have no idea whether this
> > > breaks
> > > the HW interrupt state or not.
> > The problem is when enable bit is 0 for that irq_number,
> > "writel(d->hwirq, handler->hart_base + CONTEXT_CLAIM)" wouldn't affect
> > the hw state machine. Then this irq would enter in ack state and no
> > continues irqs could come in.
>
> Really? This means that you cannot mask an interrupt while it is being
> handled? How great...
If the completion ID does not match an interrupt source that is
currently enabled for the target, the completion is silently ignored.
So, C9xx completion depends on enable-bit.

>
> > >
> > > There is an example of this in the Apple AIC driver.
> > Thx for the tip, I think your suggestion is:
> > +++ b/drivers/irqchip/irq-sifive-plic.c
> > @@ -163,7 +163,12 @@ static void plic_irq_eoi(struct irq_data *d)
> >  {
> >         struct plic_handler *handler = this_cpu_ptr(&plic_handlers);
> >
> > -       writel(d->hwirq, handler->hart_base + CONTEXT_CLAIM);
> > +       if (irqd_irq_masked(d)) {
> > +               plic_irq_unmask(d);
> > +               writel(d->hwirq, handler->hart_base + CONTEXT_CLAIM);
> > +               plic_irq_mask(d);
>
> This looks pretty dodgy. You are relying on interrupts being globally
> masked on the CPU, I guess. It probably works today, but man, what a
> terrible HW implementation.



>
> You'll definitely have to move this into a c900-specific callback.
Yes, it's an errata.

>
>         M.
>
> --
> Without deviation from the norm, progress is not possible.

--
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/