linux-kernel - Re: [PATCH RFC 3/3] arm64: dts: qcom: x1e78100-lenovo-thinkpad-t14s: add HDMI nodes

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Message-ID: <xzw26v6aqutgau3zwm3njblf2qw2em53n45ju2yhhfh7uhyxbz@6kqvink2chld>
Date: Mon, 8 Sep 2025 12:39:45 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
Cc: Jens Glathe <jens.glathe@...schoolsolutions.biz>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Vinod Koul <vkoul@...nel.org>,
        Kishon Vijay Abraham I <kishon@...nel.org>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>, linux-arm-msm@...r.kernel.org,
        linux-phy@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 3/3] arm64: dts: qcom: x1e78100-lenovo-thinkpad-t14s:
 add HDMI nodes

On Mon, Sep 08, 2025 at 09:16:29AM +0200, Konrad Dybcio wrote:
> On 9/6/25 10:41 AM, Jens Glathe wrote:
> > On 21.08.25 15:53, Neil Armstrong wrote:
> >> The Thinkpad T14s embeds a transparent 4lanes DP->HDMI transceiver
> >> connected to the third QMP Combo PHY 4 lanes.
> >>
> > [...]
> >>   .../dts/qcom/x1e78100-lenovo-thinkpad-t14s.dtsi    | 44 ++++++++++++++++++++++
> >>   1 file changed, 44 insertions(+)
> > [...]
> >> diff --git a/arch/arm64/boot/dts/qcom/x1e78100-lenovo-thinkpad-t14s.dtsi b/arch/arm64/boot/dts/qcom/x1e78100-lenovo-thinkpad-t14s.dtsi
> >> index 4cf61c2a34e31233b1adc93332bcabef22de3f86..5b62b8c3123633360f249e3ecdc8ea23f44e8e09 100644
> >> --- a/arch/arm64/boot/dts/qcom/x1e78100-lenovo-thinkpad-t14s.dtsi
> >> +++ b/arch/arm64/boot/dts/qcom/x1e78100-lenovo-thinkpad-t14s.dtsi
> > [...]
> >> +&mdss_dp2 {
> >> +    status = "okay";
> >> +};
> >> +
> >> +&mdss_dp2_out {
> >> +    data-lanes = <0 1 2 3>;
> >> +};
> >> +
> > 
> > Hi Neil,
> > 
> > shouldn't mdss_dp2_out also have the link-frequencies property?
> > 
> > +    link-frequencies = /bits/ 64 <1620000000 2700000000 5400000000 8100000000>;
> > 
> > Or is it something the bridge already negotiates?
> 
> No, it seems like our driver falls back to HBR2 (54xx) ever since the
> driver has been made aware of this property:
> 
> commit 381518a1677c49742a85f51e8f0e89f4b9b7d297
> Author: Kuogee Hsieh <quic_khsieh@...cinc.com>
> Date:   Tue Dec 27 09:45:02 2022 -0800
> 
>     drm/msm/dp: Add capability to parser and retrieve max DP link supported rate from link-frequencies property of dp_out endpoint
> 
> Dmitry, is there any reason not to allow HBR3 by default? Is our dp
> controller/driver not smart enough not to advertise rates it can't
> support, during negotiation?

I don't remember the exact details. If I remmber correctly, there were
cases where using HBR3 resulted in a less stable signal than falling
back to HBR2.

-- 
With best wishes
Dmitry