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

[Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>]

On 9/8/25 11:39 AM, Dmitry Baryshkov wrote:
> 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@quicinc.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.

A very early revision of that series has a commit message that reads:

"""
Since it is not every platform supports HBR3 link rate, this patch
limit the DP link rate at max link rate if it is specified at DTS file.
Otherwise, the max dp link rate will be limited at HBR2 as before.
"""

Konrad