linux-kernel - Re: [PATCH v7 2/9] dt-bindings: display/msm: dp-controller: fix fallback for SM6350

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Message-ID: <qy6c2gundpbz5ixqpt2hefzfb56wcrzcaclqwg2opof4zc7lep@dpc3nv6usurk>
Date: Tue, 2 Sep 2025 07:03:50 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc: Rob Clark <robin.clark@....qualcomm.com>,
        Dmitry Baryshkov <lumag@...nel.org>,
        Abhinav Kumar <abhinav.kumar@...ux.dev>,
        Jessica Zhang <jessica.zhang@....qualcomm.com>,
        Sean Paul <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>, Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Kuogee Hsieh <quic_khsieh@...cinc.com>,
        Abel Vesa <abel.vesa@...aro.org>, Mahadevan <quic_mahap@...cinc.com>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>, linux-arm-msm@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 2/9] dt-bindings: display/msm: dp-controller: fix
 fallback for SM6350

On Mon, Sep 01, 2025 at 05:45:49AM +0200, Krzysztof Kozlowski wrote:
> On Fri, Aug 29, 2025 at 01:48:15AM +0300, Dmitry Baryshkov wrote:
> > The SM6350 doesn't have MST support, as such it is not compatible with
> > the SM8350 platform. Add new entry for SM6350 with fallback to SC7180
> > (which belongs to the same generation and also doesn't have MST
> > support).
> > 
> > Fixes: 39086151593a ("dt-bindings: display: msm: dp-controller: document SM6350 compatible")
> > Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
> > ---
> >  .../devicetree/bindings/display/msm/dp-controller.yaml     | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> > 
> > diff --git a/Documentation/devicetree/bindings/display/msm/dp-controller.yaml b/Documentation/devicetree/bindings/display/msm/dp-controller.yaml
> > index aed3bafa67e3c24d2a876acd29660378b367603a..0f814aa6f51406fdbdd7386027f88dfbacb24392 100644
> > --- a/Documentation/devicetree/bindings/display/msm/dp-controller.yaml
> > +++ b/Documentation/devicetree/bindings/display/msm/dp-controller.yaml
> > @@ -31,13 +31,25 @@ properties:
> >            - qcom,sm8650-dp
> >        - items:
> >            - enum:
> > -              - qcom,sar2130p-dp
> >                - qcom,sm6350-dp
> > +          - const: qcom,sc7180-dp
> > +
> > +      # deprecated entry for compatibility with old DT
> > +      - items:
> > +          - enum:
> > +              - qcom,sm6350-dp
> > +          - const: qcom,sm8350-dp
> > +        deprecated: true
> 
> If it is only about bindings then there is little benefit in keeping
> this, just drop this case.  However you cannot drop it from DTS, so this
> is a bit pointless.

Our plan is:
- land updated DT bindings, describing MST clocks on MST-enabled
  platforms,
- land updated DTS, adding MST clocks where applicable,
- land driver changes, keeping legacy support for non-MST DTs on
  MST-enabled SoCs

> 
> Lack of MST support is not informative enough to claim it is not
> compatible with 8350. For example if it was working fine via fallback,
> then that statement is simply not correct.
> 
> And it HAD to work fine, because there is nothing binding to
> qcom,sm6350-dp.

It is working fine since currently we don't have MST support on the
driver side (nor do we describe MST clocks in DT). It's true that the
driver will have to handle non-MST DT for SM8350. However I definitely
don't want to describe both cases in the bindings. SM6350 is not going
to be compatible with the MST-enabled SM8350 schema.

Yes, it's again a lesson of 'describe hardware from the beginning', the
same issue as we observe with several other Qualcomm cores.

-- 
With best wishes
Dmitry