Re: [PATCH V5 3/3] irqchip/sifive-plic: Fixup thead, c900-plic request_threaded_irq with ONESHOT

[Anup Patel <anup@brainfault.org>]

On Thu, Oct 28, 2021 at 8:28 PM Marc Zyngier <maz@kernel.org> wrote:
>
> On Thu, 28 Oct 2021 11:55:23 +0100,
> Nikita Shubin <nikita.shubin@maquefel.me> wrote:
> >
> > Hello Marc and Guo Ren!
> >
> > On Mon, 25 Oct 2021 11:48:33 +0100
> > Marc Zyngier <maz@kernel.org> wrote:
> >
> > > On Sun, 24 Oct 2021 02:33:03 +0100,
> > > guoren@kernel.org wrote:
> > > >
> > > > From: Guo Ren <guoren@linux.alibaba.com>
> > > >
> > > > When using "devm_request_threaded_irq(,,,,IRQF_ONESHOT,,)" in the
> > > > driver, only the first interrupt could be handled, and continue irq
> > > > is blocked by hw. Because the thead,c900-plic couldn't complete
> > > > masked irq source which has been disabled in enable register. Add
> > > > thead_plic_chip which fix up c906-plic irq source completion
> > > > problem by unmask/mask wrapper.
> > > >
> > > > 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.
> > >
> > > Given this bit of the spec...
> > >
> > > > +static void plic_thead_irq_eoi(struct irq_data *d)
> > > > +{
> > > > + struct plic_handler *handler =
> > > > this_cpu_ptr(&plic_handlers); +
> > > > + if (irqd_irq_masked(d)) {
> > > > +         plic_irq_unmask(d);
> > > > +         writel(d->hwirq, handler->hart_base +
> > > > CONTEXT_CLAIM);
> > > > +         plic_irq_mask(d);
> > > > + } else {
> > > > +         writel(d->hwirq, handler->hart_base +
> > > > CONTEXT_CLAIM);
> > > > + }
> > > > +}
> > > > +
> > >
> > > ... it isn't obvious to me why this cannot happen on an SiFive PLIC.
> >
> > This indeed happens with SiFive PLIC. I am currently tinkering with
> > da9063 RTC on SiFive Unmatched, and ALARM irq fires only once. However
> > with changes proposed by Guo Ren in plic_thead_irq_eoi, everything
> > begins to work fine.
> >
> > May be these change should be propagated to plic_irq_eoi instead of
> > making a new function ?
>
> That's my impression too. I think the T-Head defect is pretty much
> immaterial when you consider how 'interesting' the PLIC architecture
> is. Conflating EOI and masking really is a misfeature...

Unfortunately, the PLIC implementation is the same across existing
boards (except T-HEAD) so this issue is there on all existing boards.

I double checked the upcoming RISC-V AIA specification and this
problem is not there in RISC-V AIA because the interrupt claim/completion
is different.
(Refer, https://github.com/riscv/riscv-aia/releases/download/0.2-draft.27/riscv-interrupts-027.pdf)

I plan to send-out RFC PATCH for AIA soon so that we get early
feedback from everyone on LKML.

Regards,
Anup

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