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

[Marc Zyngier <maz@kernel.org>]

On Wed, 20 Oct 2021 17:08:36 +0100,
Anup Patel <anup@brainfault.org> wrote:
> 
> On Wed, Oct 20, 2021 at 8:38 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.
> 
> Yes, write to claim on a masked interrupt is certainly ignored but
> read to claim does not automatically mask the interrupt.
> 
> >
> > If that's indeed correct, then a sequence such as:
> >
> > (1) irq = read(claim)
> 
> This will return highest priority pending interrupt and clear the
> pending bit as-per RISC-V PLIC spec.
> 
> > (2) mask from the interrupt handler with the right flags so that it
> > isn't done lazily
> > (3) write(irq, claim)
> >
> > will result in an interrupt blocked in ack state (and probably no more
> > interrupt for this CPU at this priority). That would be an interesting
> > bug in the current code, but also a pretty bad architectural choice.
> 
> The interrupt claim/completion is for each interrupt and not at CPU
> level so if an interrupt is masked then only that interrupt is blocked
> for all CPUs but other interrupts can still be raised.

Do you mean that another interrupt of the same priority will be able
to be taken on *this* CPU, despite the completion being silently
ignored?

	M.

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