Re: [PATCH] irqchip/sifive-plic: drop quirk for two-cell variant

From: Icenowy Zheng <uwu@icenowy.me>
To: Marc Zyngier <maz@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	 Paul Walmsley <paul.walmsley@sifive.com>,
	Samuel Holland <samuel@sholland.org>,
	linux-kernel@vger.kernel.org,  linux-riscv@lists.infradead.org
Subject: Re: [PATCH] irqchip/sifive-plic: drop quirk for two-cell variant
Date: Wed, 23 Nov 2022 20:38:56 +0800	[thread overview]
Message-ID: <16d01eebc1693916fc74e1e75458d6c0f080cf37.camel@icenowy.me> (raw)
In-Reply-To: <86o7syoq4t.wl-maz@kernel.org>

在 2022-11-22星期二的 17:28 +0000，Marc Zyngier写道：
> On Mon, 21 Nov 2022 04:20:26 +0000,
> Icenowy Zheng <uwu@icenowy.me> wrote:
> > 
> > As the special handling of edge-triggered interrupts are defined in
> > the
> > PLIC spec, we can assume it's not a quirk, but a feature of the
> > PLIC
> > spec; thus making it a quirk and use quirk-based codepath is not so
> > necessary.
> 
> It *is* necessary.
> 
> > 
> > Move to a #interrupt-cells-based practice which will allow both
> > device
> > trees without interrupt flags and with interrupt flags work for all
> > compatible strings.
> 
> No. You're tying together two unrelated concepts:
> 
> - Edges get dropped in some implementations (and only some). You can
>   argue that the architecture allows it, but I see it is an
>   implementation bug.

As the specification allows it, it's not an implementation bug -- and
for those which do not show this problem, it's possible that it's just
all using the same trigger type (e.g. Rocket).

> 
> - The need for expressing additional information in the interrupt
>   specifier is not necessarily related to the above. Other interrupt
>   controllers use extra cells to encode the interrupt affinity, for
>   example.

I think in these situations, if the interrupt controller does not
contain any special handling for edge interrupts, we can just describe
them as level ones in SW.

> 
> I want these two things to be kept separate. Otherwise, once we get
> some fancy ACPI support for RISCV (no, please...), we'll have to redo
> the whole thing...
> 
> > In addition, this addresses a stable version DT binding violation -
> > -
> > Linux v5.19 comes with "thead,c900-plic" with #interrupt-cells
> > defined to
> > be 1 instead of 2, this commit will allow DTs that complies to
> > Linux
> > v5.19 binding work (although no such DT is devliered to the public
> > now).
> 
> *That* is what should get fixed.

Supporting all stable versions' DT binding is our promise, I think.

> 
> Thanks,
> 
>         M.
> 

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