linux-kernel - Re: [PATCH 7/8] drm/msm/dp: Retry Link Training 2 with lower pattern

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Message-ID: <b4e1ea54-ff3c-408e-8716-f48001ec9113@oss.qualcomm.com>
Date: Tue, 27 May 2025 22:49:38 +0200
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Xiangxu Yin <quic_xiangxuy@...cinc.com>
Cc: Rob Clark <robdclark@...il.com>,
        Abhinav Kumar
 <quic_abhinavk@...cinc.com>,
        Sean Paul <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Kuogee Hsieh <quic_khsieh@...cinc.com>, Vinod Koul <vkoul@...nel.org>,
        Kishon Vijay Abraham I <kishon@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski
 <brgl@...ev.pl>, quic_lliu6@...cinc.com,
        quic_fangez@...cinc.com, linux-arm-msm@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-phy@...ts.infradead.org, linux-gpio@...r.kernel.org
Subject: Re: [PATCH 7/8] drm/msm/dp: Retry Link Training 2 with lower pattern

On 12/3/24 3:07 PM, Dmitry Baryshkov wrote:
> On Tue, Dec 03, 2024 at 04:13:22PM +0800, Xiangxu Yin wrote:
>>
>>
>> On 11/29/2024 9:53 PM, Dmitry Baryshkov wrote:
>>> On Fri, 29 Nov 2024 at 09:59, Xiangxu Yin <quic_xiangxuy@...cinc.com> wrote:
>>>>
>>>> Add a mechanism to retry Link Training 2 by lowering the pattern level
>>>> when the link training #2 first attempt fails. This approach enhances
>>>> compatibility, particularly addressing issues caused by certain hub
>>>> configurations.
>>>
>>> Please reference corresponding part of the standard, describing this lowering.
>>>
>> Per DisplayPort 1.4a specification Section 3.5.1.2 and Table 3-10, while the standard doesn't explicitly define a TPS downgrade mechanism, it does specify:
> 
> Anything in DP 2.1?
> 
>> - All devices shall support TPS1 and TPS2
>> - HDR2-capable devices shall support TPS3
>> - HDR3-capable devices shall support TPS4
>> While these capabilities are explicitly defined DPCD for sink devices, source device capabilities are less strictly defined, with the minimum requirement being support for TPS1 and TPS2.
>> In QCS615 DP phy is only supporting to HBR2, we observed a critical interoperability scenario with a DP->HDMI bridge. When link training at TPS4 consistently failed, downgrading to the next lower training pattern successfully established the link and display output successfully.
> 
> Any other driver doing such TPS lowering? Or maybe we should be
> selecting TPS3 for HBR2-only devices?

Bump, this patch looks interesting and I'd like to see it revisited if
it's correct

Konrad