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

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Date:   Tue, 06 Dec 2022 11:46:11 +0800
From:   Icenowy Zheng <uwu@...nowy.me>
To:     Conor Dooley <conor@...nel.org>,
        Conor Dooley <conor.dooley@...rochip.com>
Cc:     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 16:54 +0000，Conor Dooley写道：
> 
> 
> On 5 December 2022 15:59:44 GMT, Icenowy Zheng <uwu@...nowy.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?

No, but my point is that in this situation we shouldn't use C900
compatible at all because it's no longer the vanilla C900 cores.

My assumption is that the same IP cores are the same unless specially
customized.

> 
> 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?
> > > 
> >