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

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Message-ID: <879345cd8609cddccbf7bcf230923139af320b17.camel@icenowy.me>
Date:   Mon, 05 Dec 2022 23:59:44 +0800
From:   Icenowy Zheng <uwu@...nowy.me>
To:     Conor Dooley <conor.dooley@...rochip.com>
Cc:     Conor Dooley <conor@...nel.org>, Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Marc Zyngier <maz@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Jisheng Zhang <jszhang@...nel.org>,
        Samuel Holland <samuel@...lland.org>,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        linux-riscv@...ts.infradead.org
Subject: Re: [PATCH 2/3] dt-bindings: timer: sifive,clint: add compatible
 for OpenC906

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 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?
>