Re: [PATCH 2/3] dt-bindings: timer: sifive,clint: add compatible for OpenC906

From: Conor Dooley <conor@kernel.org>
To: Icenowy Zheng <uwu@icenowy.me>,
	Conor Dooley <conor.dooley@microchip.com>
Cc: Rob Herring <robh@kernel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>,
	Marc Zyngier <maz@kernel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Daniel Lezcano <daniel.lezcano@linaro.org>,
	Jisheng Zhang <jszhang@kernel.org>,
	Samuel Holland <samuel@sholland.org>,
	linux-kernel@vger.kernel.org, devicetree@vger.kernel.org,
	linux-riscv@lists.infradead.org
Subject: Re: [PATCH 2/3] dt-bindings: timer: sifive,clint: add compatible for OpenC906
Date: Mon, 05 Dec 2022 16:54:48 +0000	[thread overview]
Message-ID: <B1B2FC9D-D971-435B-A9FD-B092DE726367@kernel.org> (raw)
In-Reply-To: <879345cd8609cddccbf7bcf230923139af320b17.camel@icenowy.me>

On 5 December 2022 15:59:44 GMT, Icenowy Zheng <uwu@icenowy.me> wrote:
>在 2022-12-05星期一的 15:05 +0000，Conor Dooley写道：
>> On Mon, Dec 05, 2022 at 07:03:17PM +0800, Icenowy Zheng wrote:
>> > 在 2022-12-05星期一的 10:36 +0000，Conor Dooley写道：
>> 
>> > > You lot all know the situation here a lot more than I do...
>> > > I don't think "letting" people use the bare "thead,c900-foo"
>> > > makes
>> > > much
>> > > sense as it gives us no chance to deal with quirks down the line.
>> > 
>> > Well, after rechecking the manual, I found it possible to handle
>> > quirks
>> > -- T-Head has a custom "mcpuid" CSR (@ RISC-V CSR 0xFC0), which can
>> > be
>> > used to retrieve some identification info of the core, including
>> > its
>> > model ID, version, etc; and the T-Head PLIC/CLINT are part of their
>> > C906 SoC design that there's another "mapbaddr" CSR that could be
>> > used
>> > to retrieve the base address of them.
>> > 
>> > So I think it okay to just use "thead,c900-clint" here, and when
>> > necessary, try to retrieve mcpuid for dealing with quirks.
>> 
>> I'm not super sure I follow. What's the relevance of "mapbaddr" here?
>> We've got a reg property, so I don't think we need "mapbaddr"?
>
>Yes, it's not relevant to us here, it's only to prove that PLIC/CLINT
>is part of C906 "Core Complex".
>
>> 
>> For "mcpuid", can you be sure that implementers will not omit setting
>> that value to something unique? I'd be happier if we were overly
>> clear
>> now rather than have some headaches later. Have I missed something?
>
>These values are set by T-Head instead of individual SoC implementers
>as a CPU CSR, and it's not for uniqueness, but it's for identification
>of the CPU core revision (thus the PLIC/CLINT that come with it).

I really am missing something here that must be obvious to you.
Let me try and explain where my gap in understanding is.
If someone takes the open cores & makes a minor tweak in the plic how does knowing mcpuid help us identify that that plic is marginally different?

I must have missed something that should be apparent and look like an eejit right now!

>
>> 
>> > > I don't think that using "thead,openc906-clint", "thead,c900-
>> > > clint"
>> > > makes all that much sense either, in case someone does something
>> > > wacky
>> > > with the open-source version of the core.
>> > > 
>> > > That leaves us with either:
>> > > "vendor,soc-clint", "thead,openc906-clint", "thead,c900-clint"
>> > > or:
>> > > "vendor,soc-clint", "thead,c900-clint"
>> > > right?
>> > > 
>> > > The first one seems like possibly the better option as you'd
>> > > kinda
>> > > expect that, in a perfect word, all of the open-source IP
>> > > implementations would share quirks etc?
>> 
>

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