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

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Date:   Tue, 30 May 2023 10:53:31 +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 22/05/2023 21:15, Kevin Hilman wrote:
> Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org> writes:
> 
>> On 16/05/2023 23:31, Kevin Hilman wrote:
>>
>>>> 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.
>>>
>>> I'll try to say a bit more about the PHY block, but in fact, it's not
>>> just about differences between SoCs. On the same SoC, 2 different PHYs
>>> may have different features/capabilities.
>>>
>>> For example, on MT8365, There are 2 PHYs: CSI0 and CSI1.  CSI0 can
>>> function as a C-PHY or a D-PHY, but CSI1 can only function as D-PHY
>>> (used as the example in the binding patch[1].)  On another related SoC,
>>> there are 3 PHYs, where CSI0 is C-D but CSI1 & CSI2 are only D.
>>>
>>> So that's why it seems (at least to me) that while we need SoC
>>> compatible, it's not enough.  We also need properties to describe
>>> PHY-specific features (e.g. C-D PHY)
>>
>> I recall the same or very similar case... It bugs me now, but
>> unfortunately I cannot find it.
>>
>>>
>>> Of course, we could rely only on SoC-specific compatibles describe this.
>>> But then driver will need an SoC-specific table with the number of PHYs
>>> and per-PHY features for each SoC encoded in the driver.  Since the
>>> driver otherwise doesn't (and shouldn't, IMHO) need to know how many
>>> PHYs are on each SoC, I suggested to Julien that perhaps the additional
>>> propery was the better solution.
>>
>> Phys were modeled as separate device instances, so you would need
>> difference in compatible to figure out which phy is it.
>>
>> Other way could be to create device for all phys and use phy-cells=1.
>> Whether it makes sense, depends on the actual datasheet - maybe the
>> split phy per device is artificial? There is one PHY block with two
>> address ranges for each PHY - CSI0 and CSI1 - but it is actually one
>> block? You should carefully check this because once design is chosen,
>> you won't be able to go back to other and it might be a problem (e.g.
>> there is some top-level block for powering on all CSI instances).
> 
> We're pretty sure these are multiple instances of the IP block as they
> can operate completely independently. 
> 
>>>
>>> To me it seems redundant to have the driver encode PHYs-per-SoC info,
>>> when the per-SoC DT is going to have the same info, so my suggestion was
>>> to simplify the driver and have this kind of hardware description in the
>>> DT, and keep the driver simple, but we are definitely open to learning
>>> the "right way" of doing this.
>>
>> The property then is reasonable. It should not be bool, though, because
>> it does not scale. There can be next block which supports only D-PHY on
>> CSI0 and C-PHY on CSI1? Maybe some enum or list, depending on possible
>> configurations.
> 
> OK, looks like include/dt-bindings/phy/phy.y already has
> 
>   #define PHY_TYPE_DPHY		10
>   #define PHY_TYPE_CPHY		11
> 
> we'll add a PHY_TYPE_CDPHY and use that.   Sound reasonable?

Yes. Currently it is usually used as phy-cells argument (after the phy
number/lane/ID), but cdns,phy-type and intel,phy-mode use it directly as
property in provider. In both cases they have a bit different meaning
than yours. You want to list all supported modes or narrow/restrict
them. Maybe hisilicon,fixed-mode fits your purpose?

Best regards,
Krzysztof