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

[Guo Ren]

On Wed, Oct 20, 2021 at 9:34 PM Marc Zyngier <maz at kernel.org> wrote:
>
> On Tue, 19 Oct 2021 14:27:02 +0100,
> Guo Ren <guoren at kernel.org> wrote:
> >
> > 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.
>
> 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.

Here is the description of Interrupt Completion in PLIC spec [1]:

The PLIC signals it has completed executing an interrupt handler by
writing the interrupt ID it received from the claim to the claim/complete
register. The PLIC does not check whether the completion ID is the same
as the last claim ID for that target. If the completion ID does not match
an interrupt source that is currently enabled for the target, the
                                      ^^ ^^^^^^^^^ ^^^^^^^
completion is silently ignored.

[1] https://github.com/riscv/riscv-plic-spec/blob/master/riscv-plic.adoc

Did we misunderstand the PLIC spec?

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

--
Best Regards
 Guo Ren

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