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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Date:   Mon, 25 Sep 2023 12:44:35 +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 12:27, Herve Codina wrote:
> On Mon, 25 Sep 2023 10:21:15 +0200
> Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org> wrote:
> 
>> On 25/09/2023 10:17, Herve Codina wrote:
>>> Hi Krzysztof,
>>>
>>> On Sat, 23 Sep 2023 19:39:49 +0200
>>> Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org> wrote:
>>>   
>>>> On 22/09/2023 09:58, Herve Codina wrote:  
>>>>> The QMC (QUICC mutichannel controller) is a controller present in some
>>>>> PowerQUICC SoC such as MPC885.
>>>>> The QMC HDLC uses the QMC controller to transfer HDLC data.
>>>>>
>>>>> Additionally, a framer can be connected to the QMC HDLC.
>>>>> If present, this framer is the interface between the TDM bus used by the
>>>>> QMC HDLC and the E1/T1 line.
>>>>> The QMC HDLC can use this framer to get information about the E1/T1 line
>>>>> and configure the E1/T1 line.
>>>>>
>>>>> Signed-off-by: Herve Codina <herve.codina@...tlin.com>
>>>>> ---
>>>>>  .../soc/fsl/cpm_qe/fsl,cpm1-scc-qmc.yaml      | 24 +++++++++++++++++++
>>>>>  1 file changed, 24 insertions(+)
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/soc/fsl/cpm_qe/fsl,cpm1-scc-qmc.yaml b/Documentation/devicetree/bindings/soc/fsl/cpm_qe/fsl,cpm1-scc-qmc.yaml
>>>>> index 82d9beb48e00..61dfd5ef7407 100644
>>>>> --- a/Documentation/devicetree/bindings/soc/fsl/cpm_qe/fsl,cpm1-scc-qmc.yaml
>>>>> +++ b/Documentation/devicetree/bindings/soc/fsl/cpm_qe/fsl,cpm1-scc-qmc.yaml
>>>>> @@ -101,6 +101,27 @@ patternProperties:
>>>>>            Channel assigned Rx time-slots within the Rx time-slots routed by the
>>>>>            TSA to this cell.
>>>>>  
>>>>> +      compatible:
>>>>> +        const: fsl,qmc-hdlc    
>>>>
>>>> Why this is not a device/SoC specific compatible?  
>>>
>>> This compatible is present in a QMC channel.
>>> The parent node (the QMC itself) contains a compatible with device/SoC:
>>> --- 8< ---
>>>   compatible:
>>>     items:
>>>       - enum:
>>>           - fsl,mpc885-scc-qmc
>>>           - fsl,mpc866-scc-qmc
>>>       - const: fsl,cpm1-scc-qmc
>>> --- 8< ---
>>>
>>> At the child level (ie QMC channel), I am not sure that adding device/SoC
>>> makes sense. This compatible indicates that the QMC channel is handled by
>>> the QMC HDLC driver.
>>> At this level, whatever the device/SoC, we have to be QMC compliant.
>>>
>>> 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.

Best regards,
Krzysztof