linux-kernel - Re: [PATCH v6 3/6] irqchip: RISC-V per-HART local interrupt controller driver

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Date:   Sun, 31 May 2020 15:36:08 +0530
From:   Anup Patel <anup@...infault.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Anup Patel <anup.patel@....com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Atish Patra <atish.patra@....com>,
        Alistair Francis <Alistair.Francis@....com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>
Subject: Re: [PATCH v6 3/6] irqchip: RISC-V per-HART local interrupt
 controller driver

On Sun, May 31, 2020 at 3:03 PM Marc Zyngier <maz@...nel.org> wrote:
>
> On 2020-05-31 06:36, Anup Patel wrote:
> > On Sat, May 30, 2020 at 5:31 PM Marc Zyngier <maz@...nel.org> wrote:
>
> [...]
>
> >> >       plic_set_threshold(handler, PLIC_DISABLE_THRESHOLD);
> >>
> >> Why do you need to both disable the interrupt *and* change the
> >> priority
> >> threshold? It seems to be that one of them should be enough, but my
> >> kno9wledge of the PLIC is limited. In any case, this would deserve a
> >> comment.
> >
> > Okay, I will test and remove "disable the interrupt" part from
> > plic_dying_cpu().
>
> Be careful, as interrupt enabling/disabling is refcounted in order
> to allow nesting. If you only enable on CPU_ON and not disable
> on CPU_OFF, you will end-up with a depth that only increases,
> up to the point where you hit the roof (it will take a while though).
>
> I would keep the enable/disable as is, and drop the priority
> setting from the CPU_OFF path.

The PLIC threshold is like GICv2 CPU interface enable/disable.

Based on your comment, we should only program the PLIC threshold
in CPU_ON and don't touch the PLIC threshold in CPU_OFF. Right??

>
> >> >       return 0;
> >> > @@ -260,7 +266,11 @@ static int plic_starting_cpu(unsigned int cpu)
> >> >  {
> >> >       struct plic_handler *handler = this_cpu_ptr(&plic_handlers);
> >> >
> >> > -     csr_set(CSR_IE, IE_EIE);
> >> > +     if (plic_parent_irq)
> >> > +             enable_percpu_irq(plic_parent_irq,
> >> > +                               irq_get_trigger_type(plic_parent_irq));
> >> > +     else
> >> > +             pr_warn("cpu%d: parent irq not available\n");
> >>
> >> What does it mean to carry on if the interrupt cannot be signaled?
> >> Shouldn't you error out instead, and leave the CPU dead?
> >
> > The CPU is not dead if we cannot enable RISC-V INTC external
> > interrupt because the Timer and IPIs interrupts are always through
> > RISC-V INTC. The PLIC external interrupt not present for a CPU
> > only means that that CPU cannot receive peripherial interrupts.
> >
> > On a sane RISC-V system, if PLIC is present then all CPUs should
> > be able to get RISC-V INTC external interrupt. Base on this rationale,
> > I have put a warning for plic_parent_irq == 0.
>
> Fair enough.
>
>          M.
> --
> Jazz is not dead. It just smells funny...

Regards,
Anup