Message-ID: <rivif67xphnva5ql3s6p6gk6uzjde55nzi34unyqktrrjtalgm@gnjqibxo6jzz>
Date: Tue, 2 Sep 2025 19:29:24 +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 Tue, Sep 02, 2025 at 02:09:02PM +0200, Krzysztof Kozlowski wrote:
> On 02/09/2025 12:01, Dmitry Baryshkov wrote:
> > On Tue, Sep 02, 2025 at 08:04:54AM +0200, Krzysztof Kozlowski wrote:
> >> On 02/09/2025 06:03, Dmitry Baryshkov wrote:
> >>> 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,
> >>
> >> This part breaks all out-of-tree users of DTS.
> > 
> > User of which one? SM6350 or all DTS?
> 
> SM6350.
> 
> > 
> > It extends the ABI, so no, it shouldn't. We add regions and clocks,
> 
> No, it does not extend the ABI. You are changing the fallback, so you
> are changing the ABI.


I was thinking about the whole patchset: adding DP MST-specific clocks
and regions. For this particular patch you are correct.

> 
> 
> > existing users can use the previously defined feature set.
> > 
> > Anyway, is it about adding more explicit note to the cover letter and/or
> > commit message or would you have any other proposal on how to handle the
> > issue?
> 
> Please be explicit if Linux was working with the old fallback or not. In
> the DTS change be explicit about impact, e.g. that sc7180 fallback is
> there since forever.

Ack.

> 
> > 
> >>
> >>> - 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.
> >>
> >> The question is rather: is SM6350 going to be compatible (working) with
> >> MST-enabled drivers.
> > 
> > If somebody implements e.g. U-Boot DP MST support after we land DP MST
> > bindings, I would not guarantee that. SM8350 will already have second
> > stream clock.
> 
> 
> Having second stream clock does not mean device stops working with Linux
> kernel. I can easily imagine both cases after adding MST to the drivers,
> the SM6350 stops working or keeps working...

It will keep on working, since we need to support old MST-less
sm8350.dtsi. 

-- 
With best wishes
Dmitry

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