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

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Date:   Mon, 1 Jun 2020 17:33:47 +0800
From:   Guo Ren <guoren@...nel.org>
To:     Anup Patel <anup@...infault.org>
Cc:     Marc Zyngier <maz@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Jason Cooper <jason@...edaemon.net>,
        Anup Patel <anup.patel@....com>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        Atish Patra <atish.patra@....com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Alistair Francis <Alistair.Francis@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-riscv <linux-riscv@...ts.infradead.org>
Subject: Re: [PATCH v6 3/6] irqchip: RISC-V per-HART local interrupt
 controller driver

Hi Anup,

On Mon, Jun 1, 2020 at 12:09 PM Anup Patel <anup@...infault.org> wrote:
>
> On Sun, May 31, 2020 at 4:23 PM Marc Zyngier <maz@...nel.org> wrote:
> >
> > On 2020-05-31 11:06, Anup Patel wrote:
> > > 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.
> >
> > Looking at the documentation[1], that's not really a correct analogy:
> >
> > - The PLIC is far removed from the CPU, and is more akin a GICv3
> >    distributor. The INTC itself is more like a GICv3 redistributor,
> >    as it deals with local interrupts only. I don't see anything
> >    in the RISC-V architecture that actually behaves like a GIC
> >    CPU interface (not necessarily a bad thing...).
> >
> > - The threshold register is not an ON/OFF, but a priority mask,
> >    similar to the GIC PMR (except that the PMR lives in the CPU
> >    interface and affects all interrupts targetting this CPU while
> >    this only seem to affect PLIC interrupts and not the INTC interrupts).
> >    You may be using it as an ON/OFF for now as you don't support
> >    multiple priorities yet, but that's just a SW thing.
>
> Yes, your analogy is correct.
>
> BTW, PLIC does not handle MSI and does not have virtualization support
> pass-through interrupts. We will most likely see a new RISC-V interrupt
> controller spec for these capabilities.
>
> Also, the PLIC spec is now owned by RISC-V foundation (not SiFive) so
> we will have to rename the driver to "irq-riscv-plic" and will have a new
> generic compatible string "riscv,plic-1.0.0". One of us (me or Palmer) will
> send separate patches for this renaming. I hope you will be fine with this??
> (Refer, https://github.com/riscv/riscv-plic-spec)

That's great, we follow riscv-plic in hardware, but don't want to use
sifive string in dts.

Acked & Thx for the job.

-- 
Best Regards
 Guo Ren

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