linux-kernel - Re: [PATCH v2 1/2] dt-bindings: phy: add mediatek mipi csi driver v 0.5

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Message-ID: <fbf1b0a6-f45d-69a0-5de6-8269567e15b3@linaro.org>
Date:   Tue, 16 May 2023 19:46:40 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Kevin Hilman <khilman@...libre.com>,
        Julien Stephan <jstephan@...libre.com>
Cc:     robh@...nel.org, chunkuang.hu@...nel.org,
        linux-mediatek@...ts.infradead.org,
        Florian Sylvestre <fsylvestre@...libre.com>,
        Chunfeng Yun <chunfeng.yun@...iatek.com>,
        Andy Hsieh <andy.hsieh@...iatek.com>,
        Vinod Koul <vkoul@...nel.org>,
        Kishon Vijay Abraham I <kishon@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        "moderated list:ARM/Mediatek USB3 PHY DRIVER" 
        <linux-arm-kernel@...ts.infradead.org>,
        "open list:GENERIC PHY FRAMEWORK" <linux-phy@...ts.infradead.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] dt-bindings: phy: add mediatek mipi csi driver v
 0.5

On 16/05/2023 19:00, Kevin Hilman wrote:
>>>>> +  compatible:
>>>>> +    enum:
>>>>> +      - mediatek,phy-mipi-csi-0-5
>>>>
>>>> SoC based compatibles. 0-5 is odd.
>>>>
>>>>> +
>>>>> +  reg:
>>>>> +    maxItems: 1
>>>>> +
>>>>> +  '#phy-cells':
>>>>> +    const: 0
>>>>> +
>>>>> +  mediatek,is_cdphy:
>>>>
>>>> No underscores in node names.
>>>>
>>>>> +    description:
>>>>> +      Specify if the current phy support CDPHY configuration
>>>>
>>>> Why this cannot be implied from compatible? Add specific compatibles.
>>>>
>>>>
>>> This cannot be implied by compatible because the number of phys depends
>>> on the soc and each phy can be either D-PHY only or CD-PHY capable.
>>> For example mt8365 has 2 phy: CSI0 and CSI1. CSI1 is DPHY only and CSI0 is CD-PHY
>>
>> So it is SoC specific so why it cannot be implied by compatible? I don't
>> understand. You will have SoC specific compatibles, right? or you just
>> ignored my comments here?
> 
> Julien, I think you had SoC specific compatibles in an earlier version
> but then changed it to be generic based on reviewer feedback.  However,
> that earlier version of the driver was trying to do a bunch of SoC
> specific logic internally and support multiple SoCs.  You've now greatly
> simplified the driver, with only a few SoC specific decisions needed.
> These can be implied by the driver based SoC specific compatible, as
> Krzysztof suggests, so you should just go back to having SoC specific
> compatibles.
> 

Yes. If there is common part, e.g. several SoCs use the same device with
same programming model, then the generic recommendation is to have
SoC-based fallback (used also in the driver) and SoC-specific compatibles.

Second accepted solution is to have generic fallback which does not use
SoC in the compatible (and of course mandatory SoC-specific comaptibles).

Third is to use versioned IP blocks.

The second case also would work, if it is applicable to you (you really
have fallback matching all devices). Third solution depends on your
versioning and Rob expressed dislike about it many times.

We had many discussions on mailing lists, thus simplifying the review -
I recommend the first choice. For a better recommendation you should say
a bit more about the block in different SoCs.

Best regards,
Krzysztof