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

[Darius Rad <darius@bluespec.com>]

On Wed, Oct 20, 2021 at 09:48:36PM +0530, Anup Patel wrote:
> On Wed, Oct 20, 2021 at 8:29 PM Darius Rad <darius@bluespec.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@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.
> > >
> > > 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. ...

You mean is in the process of adopting it, right?

> ... 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.
> 

I would expect the review process to produce a proper specification, rather
than a verbatim copy of the SiFive datasheet, and clarify some ambgiuous
and implementation specific language.  Clarifying the specification does
not necessarily make all existing implementations non-compliant, as this
has been done numerous times with other RISC-V specifications.

> 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
> 
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