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

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Message-ID: <CAAhSdy1FS+rTO8JWfqKVMLPBUOzmy0d5D1s=psfxbm4s6QrBCA@mail.gmail.com>
Date:   Wed, 20 Oct 2021 21:48:36 +0530
From:   Anup Patel <anup@...infault.org>
To:     Guo Ren <guoren@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Samuel Holland <samuel@...lland.org>,
        Anup Patel <anup@...infault.org>,
        Atish Patra <atish.patra@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Palmer Dabbelt <palmer@...belt.com>,
        Heiko Stübner <heiko@...ech.de>,
        Rob Herring <robh@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [PATCH V4 1/3] irqchip/sifive-plic: Add thead,c900-plic support

On Wed, Oct 20, 2021 at 8:29 PM Darius Rad <darius@...espec.com> wrote:
>
> On Wed, Oct 20, 2021 at 10:19:06PM +0800, Guo Ren wrote:
> > On Wed, Oct 20, 2021 at 9:34 PM Marc Zyngier <maz@...nel.org> wrote:
> > >
> > > On Tue, 19 Oct 2021 14:27:02 +0100,
> > > Guo Ren <guoren@...nel.org> wrote:
> > > >
> > > > On Tue, Oct 19, 2021 at 6:18 PM Marc Zyngier <maz@...nel.org> wrote:
> > > > >
> > > > > On Tue, 19 Oct 2021 10:33:49 +0100,
> > > > > Guo Ren <guoren@...nel.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?
> >
>
> That clause sounds to me like it is due to the SiFive implementation, which
> the RISC-V PLIC specification is based on.  Since the PLIC spec is still a
> draft I would expect it to change before release.

The SiFive PLIC has been adopted by various RISC-V platforms (including
SiFive themselves). Almost all existing RISC-V boards have PLIC as the
interrupt controller.

Considering the wide usage of PLIC across existing platforms, the RISC-V
International has adopted it as an official RISC-V non-ISA spec. Of course,
the RISC-V PLIC spec needs to follow the process for RISC-V non-ISA spec
but changing the RISC-V PLIC spec now would mean all existing RISC-V
platforms will become non-compliant.

The RISC-V AIA spec is intended to replace the RISC-V PLIC spec as the
new interrupt controller spec for future RISC-V platforms.

Regards,
Anup