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

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Message-ID: <7hcz2snpnw.fsf@baylibre.com>
Date:   Mon, 22 May 2023 12:15:31 -0700
From:   Kevin Hilman <khilman@...libre.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        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

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?

Kevin