linux-kernel - Re: drm/msm/dp: Introduce link training per-segment for LTTPRs

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Message-ID: <aA8yFI2Bvm-lFJTl@hovoldconsulting.com>
Date: Mon, 28 Apr 2025 09:45:24 +0200
From: Johan Hovold <johan@...nel.org>
To: Aleksandrs Vinarskis <alex.vinarskis@...il.com>
Cc: Dmitry Baryshkov <lumag@...nel.org>, linux-arm-msm@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org, dmitry.baryshkov@....qualcomm.com,
	Rob Clark <robdclark@...il.com>,
	Abhinav Kumar <quic_abhinavk@...cinc.com>,
	Sean Paul <sean@...rly.run>,
	Marijn Suijten <marijn.suijten@...ainline.org>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	laurentiu.tudor1@...l.com, abel.vesa@...aro.org
Subject: Re: drm/msm/dp: Introduce link training per-segment for LTTPRs

On Thu, Apr 17, 2025 at 04:10:31AM +0200, Aleksandrs Vinarskis wrote:
> Recently added Initial LTTPR support in msm/dp has configured LTTPR(s)
> to non-transparent mode to enable video output on X1E-based devices
> that come with LTTPR on the motherboards. However, video would not work
> if additional LTTPR(s) are present between sink and source, which is
> the case for USB Type-C docks (eg. Dell WD19TB/WD22TB4), and at least
> some universal Thunderbolt/USB Type-C monitors (eg. Dell U2725QE).

Does this mean that the incomplete LTTPR support in 6.15-rc1 broke
adapters or docks with retimers in transparent mode?

You describe at least one of this patches as a fix but I'm not seeing
any Fixes tags or indication that these need to go into 6.15-rc to fix
a regression.

> Changes in v3:
> - Split 1st patch into 3
> - Simplified handling of max_lttpr_lanes/max_lttpr_rate
> - Moved lttpr_common_caps to msm_dp_link (not msm_dp_panel, as LTTPRs
>   are link related, not panel related)
> - Picked Stefan's T-b tag (last patch only, as 1st one is getting split)
> - Droped Abel's R-b tags from 1st patch that got split due to high diff
> - Fixed alignment issues, initialization of variables, debug prints
> - Moved lttpr_count to avoid ugly pointer
> - Link to v2: https://lore.kernel.org/all/20250311234109.136510-1-alex.vinarskis@gmail.com/

I tested v2 due to the dependencies in msm-next and DP altmode still
works on my X13s and T14s (while not using any docks):

Tested-by: Johan Hovold <johan+linaro@...nel.org>

> Aleksandrs Vinarskis (4):
>   drm/msm/dp: Fix support of LTTPR initialization
>   drm/msm/dp: Account for LTTPRs capabilities
>   drm/msm/dp: Prepare for link training per-segment for LTTPRs
>   drm/msm/dp: Introduce link training per-segment for LTTPRs

Johan