linux-kernel - Re: [PATCH v6 08/30] dt-bindings: soc: fsl: cpm_qe: cpm1-scc-qmc: Add support for QMC HDLC

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Message-ID: <e8ee6529-b194-4588-96c0-1459f214d005@linaro.org>
Date:   Tue, 26 Sep 2023 22:59:14 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Herve Codina <herve.codina@...tlin.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Andrew Lunn <andrew@...n.ch>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>, Lee Jones <lee@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Qiang Zhao <qiang.zhao@....com>, Li Yang <leoyang.li@....com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>,
        Shengjiu Wang <shengjiu.wang@...il.com>,
        Xiubo Li <Xiubo.Lee@...il.com>,
        Fabio Estevam <festevam@...il.com>,
        Nicolin Chen <nicoleotsuka@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Randy Dunlap <rdunlap@...radead.org>, netdev@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, alsa-devel@...a-project.org,
        Simon Horman <horms@...nel.org>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH v6 08/30] dt-bindings: soc: fsl: cpm_qe: cpm1-scc-qmc: Add
 support for QMC HDLC

On 25/09/2023 15:50, Herve Codina wrote:
>>>>> With these details, do you still think I need to change the child (channel)
>>>>> compatible ?    
>>>>
>>>> From OS point of view, you have a driver binding to this child-level
>>>> compatible. How do you enforce Linux driver binding based on parent
>>>> compatible? I looked at your next patch and I did not see it.  
>>>
>>> We do not need to have the child driver binding based on parent.  
>>
>> Exactly, that's what I said.
>>
>>> We have to ensure that the child handles a QMC channel and the parent provides
>>> a QMC channel.
>>>
>>> A QMC controller (parent) has to implement the QMC API (include/soc/fsl/qe/qmc.h)
>>> and a QMC channel driver (child) has to use the QMC API.  
>>
>> How does this solve my concerns? Sorry, I do not understand. Your driver
>> is a platform driver and binds to the generic compatible. How do you
>> solve regular compatibility issues (need for quirks) if parent
>> compatible is not used?
>>
>> How does being QMC compliant affects driver binding and
>> compatibility/quirks?
>>
>> We are back to my original question and I don't think you answered to
>> any of the concerns.
> 
> Well, to be sure that I understand correctly, do you mean that I should
> provide a compatible for the child (HDLC) with something like this:
> --- 8< ---
>   compatible:
>     items:
>       - enum:
>           - fsl,mpc885-qmc-hdlc
>           - fsl,mpc866-qmc-hdlc
>       - const: fsl,cpm1-qmc-hdlc
>       - const: fsl,qmc-hdlc
> --- 8< ---

Yes, more or less, depending on actual compatibility and SoC-family.
Maybe "fsl,cpm1-qmc-hdlc" item in the middle is not needed.

> 
> If so, I didn't do that because a QMC channel consumer (driver matching
> fsl,qmc-hdlc) doesn't contains any SoC specific part.

Just like hundreds of other drivers. :)

There is a paragraph about specific compatibles here:
https://www.kernel.org/doc/html/latest/devicetree/bindings/writing-schema.html


> It uses the channel as a communication channel to send/receive HDLC frames
> to/from this communication channel.
> All the specific SoC part is handled by the QMC controller (parent) itself and
> not by any consumer (child).

OK, so you guarantee in 100% for this hardware and all future (including
designs unknown currently), that they will be 100% compatible with
existing QMC channel consumer (child, matching fsl,qmc-hdlc) driver,
thus there will be no need for any quirk. Specifically, there will be no
chances that it would be reasonable to re-use the same driver for child
(currently fsl,qmc-hdlc) in different parent.

P.S. If you received this email twice, apologies, I have here troubles
with internet.

Best regards,
Krzysztof