linux-kernel - Re: [PATCH v1 17/17] drm/mediatek: Add mt8195-dpi support to drm

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Message-ID: <CABnWg9sPuJQQOmLGDSgE86NCRh_3GqWZ0dFAs_J2PO9HCVbExQ@mail.gmail.com>
Date:   Wed, 5 Oct 2022 02:34:19 -0700
From:   Guillaume Ranquet <granquet@...libre.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Chun-Kuang Hu <chunkuang.hu@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Kishon Vijay Abraham I <kishon@...com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Vinod Koul <vkoul@...nel.org>, Stephen Boyd <sboyd@...nel.org>,
        David Airlie <airlied@...ux.ie>,
        Rob Herring <robh+dt@...nel.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Chunfeng Yun <chunfeng.yun@...iatek.com>,
        CK Hu <ck.hu@...iatek.com>, Jitao shi <jitao.shi@...iatek.com>
Cc:     linux-mediatek@...ts.infradead.org,
        dri-devel@...ts.freedesktop.org,
        Pablo Sun <pablo.sun@...iatek.com>, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Mattijs Korpershoek <mkorpershoek@...libre.com>,
        linux-arm-kernel@...ts.infradead.org,
        linux-phy@...ts.infradead.org, devicetree@...r.kernel.org
Subject: Re: [PATCH v1 17/17] drm/mediatek: Add mt8195-dpi support to drm_drv

On Tue, 04 Oct 2022 17:05, Krzysztof Kozlowski
<krzysztof.kozlowski@...aro.org> wrote:
>On 04/10/2022 13:55, Guillaume Ranquet wrote:
>>> No. You said what the code is doing. I think I understand this. You
>>> still do not need more compatibles. Your sentence did not clarify it
>>> because it did not answer at all to question "why". Why do you need it?
>>>
>>> Sorry, the change looks not correct.
>>>
>>> Best regards,
>>> Krzysztof
>>>
>>
>> I need a new compatible to adress the specifics of mt8195 in the mtk_dpi driver,
>> the change is in this series with:
>> [PATCH v1 16/17] drm/mediatek: dpi: Add mt8195 hdmi to DPI driver [1]
>
>But you do not have specifics of mt8195. I wrote it in the beginning.
>
>>
>> I then need to add that compatible to the "list" here in mtk_drm_drv.
>
>No, you do not... I checked the driver and there is no single need... or
>convince me you need.
>
>> I don't see a way around this unless I rewrite the way mtk_drm_drv works?
>
>Why rewrite? You have all compatibles in place.
>
>>
>> Maybe if I declare a new compatible that is generic to all mediatek
>> dpi variants?
>
>You were asked to use fallback. Don't create some fake fallbacks. Use
>existing ones.
>
>> and have all the dts specify the node with both the generic dpi and
>> the specific compatible?
>>
>> dpi@xxx {
>> 	compatible = "mediatek,dpi", "mediatek,mt8195-dpi";
>
>I don't know what's this but certainly looks odd. Some wild-card
>compatible in front (not fallback) and none are documented.
>
>> 	...
>> }
>>
>> Then I can "collapse" all the dpi related nodes in mtk_drm_drv under
>> "mediatek,dpi" ?
>>
>> I guess would have to do the change for all other components that are needed in
>> mtk_drm_drv (mmsys, aal, ccor, color, dither, dsc, gamma, mutex...).
>>
>> That's the only trivial way I can think of implementing this with the
>> current status
>> of the mtk_drm stack.
>>
>> Do you have any other ideas in mind?
>
>Use fallback of compatible device. That's the common pattern.
>Everywhere, Mediatek as well.
>

I'm unsure about what a "fallback of compatible device" is.
But from what I can gather, you'd have me write the dts as:

dpi@xxx {
	compatible = "mediatek,mt8195-dpi", "mediatek,mt2701-dpi";
	...
}

so that the mtk_dpi driver will use the specific mt8195 dpi config and
the mtk_drm driver will fallback to mt2701 to find the compatible it needs.

Would you like me to use this pattern for all the other compatibles declared
in the mtk_ddp_comp_ids array in separate patches?

Thx,
Guillaume.

>Best regards,
>Krzysztof
>