linux-kernel - Re: [PATCH RFC/WIP 1/4] arm64: dts: qcom: sm8750: Add display (MDSS) with Display CC

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Message-ID: <CAO9ioeV0H9OnmHke2prWSqB8+oD4PRfD0ph7qPahwKJHOPpOzg@mail.gmail.com>
Date: Sun, 4 May 2025 00:01:45 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Abhinav Kumar <quic_abhinavk@...cinc.com>
Cc: Konrad Dybcio <konrad.dybcio@....qualcomm.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>, Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Jessica Zhang <jesszhan@...cinc.com>,
        Abhinav Kumar <abhinavk@...cinc.com>, Abel Vesa <abel.vesa@...aro.org>,
        linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC/WIP 1/4] arm64: dts: qcom: sm8750: Add display (MDSS)
 with Display CC

On Sat, 3 May 2025 at 22:59, Abhinav Kumar <quic_abhinavk@...cinc.com> wrote:
>
>
>
> On 5/2/2025 10:51 PM, Dmitry Baryshkov wrote:
> > On Tue, Apr 29, 2025 at 04:07:24PM -0700, Abhinav Kumar wrote:
> >>
> >>
> >> On 4/28/2025 2:31 PM, Konrad Dybcio wrote:
> >>> On 4/24/25 3:04 PM, Krzysztof Kozlowski wrote:
> >>>> Add device nodes for entire display: MDSS, DPU, DSI, DSI PHYs,
> >>>> DisplayPort and Display Clock Controller.
> >>>>
> >>>> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
> >>>>
> >>>> ---
> >>>
> >>> [...]
> >>>
> >>>> +                          mdp_opp_table: opp-table {
> >>>> +                                  compatible = "operating-points-v2";
> >>>> +
> >>>
> >>> The computer tells me there's also a 156 MHz rate @ SVS_D1
> >>>
> >>> Maybe Abhinav could chime in whether we should add it or not
> >>>
> >>
> >> Yes I also see a 156Mhz for LOW_SVS_D1 but we had a similar entry even for
> >> sm8650 and did not publish it in the dt.
> >>
> >> It was present till sm8450.dtsi but dropped in sm8550/sm8650 even though
> >> LOW_SVS_D1 is present even on those.
> >>
> >> I think the reason could be that the displays being used on the reference
> >> boards will need a pixel clock of atleast >= low_svs and the MDP clock
> >> usually depends on the value of the DSI pixel clock (which has a fixed
> >> relationship to the byte clock) to maintain the data rate. So as a result
> >> perhaps even if we add it, for most displays this level will be unused.
> >>
> >> If we end up using displays which are so small that the pixel clock
> >> requirement will be even lower than low_svs, we can add those.
> >>
> >> OR as an alternative, we can leave this patch as it is and add the
> >> low_svs_d1 for all chipsets which support it together in another series that
> >> way it will have the full context of why we are adding it otherwise it will
> >> look odd again of why sm8550/sm8650 was left out but added in sm8750.
> >
> > I think it's better to describe hardware accurately, even if the
> > particular entry ends up being unused. I'd vote for this option.
> >
> >>> [...]
> >>>
> >>>> +                          mdss_dsi_opp_table: opp-table {
> >>>> +                                  compatible = "operating-points-v2";
> >>>> +
> >>>
> >>> Similarly there's a 140.63 MHz rate at SVS_D1, but it seems odd
> >>> with the decimals
> >>
> >> For this one, yes its true that LOW_SVS_D1 is 140.63Mhz for sm8750 but this
> >> voltage corner was somehow never used for DSI byte clock again I am thinking
> >> this is because for the display resolutions we use, we will always be >=
> >> low_svs so the low_svs_d1 will never hit even if we add it.
> >
> > Please add all voltage/frequency corners. Think about low-res DP or
> > low-res, low-rate WB.
> >
>
> Sounds good, lets go ahead and add all the voltage/freq corners.
>
> Like I noted, even for sm8550/sm8650 the low_svs_d1 was missed out, so
> if we are adding it for sm8750 now in this series, a follow up patch
> should also be sent to add them for sm8550/sm8650 as well. That way we
> will fix them all up together and this does not come across as a
> discrepancy.

Abhinav, if you know a missing piece, please send a patch, fixing it.

-- 
With best wishes
Dmitry