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

[Guo Ren <guoren@kernel.org>]

On Thu, Oct 21, 2021 at 4:33 PM Marc Zyngier <maz@kernel.org> wrote:
>
> On Thu, 21 Oct 2021 03:00:43 +0100,
> Guo Ren <guoren@kernel.org> wrote:
> >
> > On Wed, Oct 20, 2021 at 11:08 PM Marc Zyngier <maz@kernel.org> wrote:
> > >
> > > On Wed, 20 Oct 2021 15:33:49 +0100,
> > > Anup Patel <anup@brainfault.org> wrote:
> > > >
> > > > On Wed, Oct 20, 2021 at 7:04 PM Marc Zyngier <maz@kernel.org> wrote:
> > > > >
> > > > > On Tue, 19 Oct 2021 14:27:02 +0100,
> > > > > Guo Ren <guoren@kernel.org> wrote:
> > > > > >
> > > > > > On Tue, Oct 19, 2021 at 6:18 PM Marc Zyngier <maz@kernel.org> wrote:
> > > > > > >
> > > > > > > On Tue, 19 Oct 2021 10:33:49 +0100,
> > > > > > > Guo Ren <guoren@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.
> > > > >
> > > > > Is that what the PLIC spec says? Or what your implementation does? I
> > > > > can understand that one implementation would be broken, but if the
> > > > > PLIC architecture itself is broken, that's far more concerning.
> > > >
> > > > Yes, we are dealing with a broken/non-compliant PLIC
> > > > implementation.
> > > >
> > > > The RISC-V PLIC spec defines a very different behaviour for the
> > > > interrupt claim (i.e. readl(claim)) and interrupt completion (i.e.
> > > > writel(claim)). The T-HEAD PLIC implementation does things
> > > > different from what the RISC-V PLIC spec says because it will
> > > > mask an interrupt upon interrupt claim whereas PLIC spec says
> > > > it should only clear the interrupt pending bit (not mask the interrupt).
> > > >
> > > > Quoting interrupt claim process (chapter 9) from PLIC spec:
> > > > "The PLIC can perform an interrupt claim by reading the claim/complete
> > > > register, which returns the ID of the highest priority pending interrupt or
> > > > zero if there is no pending interrupt. A successful claim will also atomically
> > > > clear the corresponding pending bit on the interrupt source."
> > > >
> > > > Refer, https://github.com/riscv/riscv-plic-spec/blob/master/riscv-plic.adoc
> > >
> > > That's not the point I'm making. According to Guo, the PLIC (any
> > > implementation of it) will ignore a write to claim on a masked
> > > interrupt.
> > >
> > > If that's indeed correct, then a sequence such as:
> > >
> > > (1) irq = read(claim)
> > > (2) mask from the interrupt handler with the right flags so that it
> > > isn't done lazily
> > > (3) write(irq, claim)
> >
> > How about letting the IRQ chip change?
> >
> > diff --git a/kernel/irq/chip.c b/kernel/irq/chip.c
> > index a98bcfc4be7b..ed6ace1058ac 100644
> > --- a/kernel/irq/chip.c
> > +++ b/kernel/irq/chip.c
> > @@ -444,10 +444,10 @@ void unmask_threaded_irq(struct irq_desc *desc)
> >  {
> >         struct irq_chip *chip = desc->irq_data.chip;
> >
> > +       unmask_irq(desc);
> > +
> >         if (chip->flags & IRQCHIP_EOI_THREADED)
> >                 chip->irq_eoi(&desc->irq_data);
> > -
> > -       unmask_irq(desc);
> >  }
> >
> >  /*
> > @@ -673,8 +673,8 @@ static void cond_unmask_eoi_irq(struct irq_desc
> > *desc, struct irq_chip *chip)
> >          */
> >         if (!irqd_irq_disabled(&desc->irq_data) &&
> >             irqd_irq_masked(&desc->irq_data) && !desc->threads_oneshot) {
> > -               chip->irq_eoi(&desc->irq_data);
> >                 unmask_irq(desc);
> > +               chip->irq_eoi(&desc->irq_data);
> >         } else if (!(chip->flags & IRQCHIP_EOI_THREADED)) {
> >                 chip->irq_eoi(&desc->irq_data);
> >         }
>
> No, I don't think that's acceptable, and I strongly suspect that other
> irqchips have the opposite requirement. You'll have to keep the
> workaround in the PLIC code and track the EOI vs unmask to do the
> right thing in both callbacks.
Okay...

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



-- 
Best Regards
 Guo Ren

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