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

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Message-ID: <04BFA351-92E6-442F-B065-9C287C174A0E@kernel.org>
Date:   Tue, 06 Dec 2022 06:33:13 +0000
From:   Conor Dooley <conor@...nel.org>
To:     Icenowy Zheng <uwu@...nowy.me>,
        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



On 6 December 2022 03:46:11 GMT, Icenowy Zheng <uwu@...nowy.me> wrote:
>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.

Ah see that is assuming people get it right.
I've been in the mindset of "what if the difference is only noticed after the DT has been shipped".
I guess that's where we've been at odds.

>
>> 
>> 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?
>> > > 
>> > 
>