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

Rob Clark
2025-04-24T21:12:29Z

On Wed, Apr 16, 2025 at 7:13 PM Aleksandrs Vinarskis
<alex.vinarskis@gmail.com> 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).
>
> First, take into account LTTPR capabilities when computing max link
> rate, number of lanes. Take into account previous discussion on the
> lists - exit early if reading DPCD caps failed. This also fixes
> "*ERROR* panel edid read failed" on some monitors which seems to be
> caused by msm_dp_panel_read_sink_caps running before LTTPR(s) are
> initialized.
>
> Finally, implement link training per-segment. Pass lttpr_count to all
> required helpers.
> This seems to also partially improve UI (Wayland) hanging when
> changing external display's link parameters (resolution, framerate):
> * Prior to this series, via direct USB Type-C to display connection,
> attempt to change resolution or framerate hangs the UI, setting does
> not stick. Some back and forth replugging finally sets desired
> parameters.
> * With this series, via direct USB Type-C to display connection,
> changing parameters works most of the time, without UI freezing. Via
> docking station/multiple LTTPRs the setting again does not stick.
> * On Xorg changing link paramaters works in all combinations.
>
> These appear to be mainlink initialization related, as in all cases LT
> passes successfully.
>
> Test matrix:
> * Dell XPS 9345, Ubuntu 24.10, Gnome 47, Wayland (myself)
> * Left USB Type-C, Right USB Type-C
> * Direct monitor connection, Dell WD19TB, Dell WD22TB4, USB
> Type-C to HDMI dongle, USB Type-C to DP dongle
> * Dell AW3423DWF, Samsung LS24A600, dual Samsung LS24A600 (one
> monitor per USB Type-C connector)
> * Dell XPS 9345, Ubuntu 24.10, Gnome 47, Wayland (myself)
> * Left USB Type-C, Right USB Type-C
> * Direct monitor connection
> * Samsung S34BG85 (USB Type-C), Dell U2725QE (universal
> Thunderbolt/USB Type-C, probes with an LTTPR when in USB
> Type-C/DP Alt mode)
> * Dell XPS 9345, Debian trixie/sid, Gnome 48, Wayland (Stefan Schmidt)
> * Left USB Type-C, Right USB Type-C
> * Dell WD15 Dock with DisplayPort connected
> * Dell HD22Q dock with HDMI connected
> * USB Type-C to HDMI dongle
> * Dell U3417W

For the series,

Tested-by: Rob Clark <robdclark@gmail.com> # yoga slim 7x

patch 4/4 had a number of conflicting hunks, you might need to rebase
on msm-next

BR,
-R

> In both cases, "Thunderbot Support"/"USB4 PCIE Tunneling" was disabled
> in UEFI to force universal Thunderbolt/USB Type-C devices to work in
> DP Alt mode.
> In both cases laptops had HBR3 patches applied [1], resulting in
> maximum successful link at 3440x1440@100hz and 4k@60hz respectively.
> When using Dell WD22TB4/U2725QE, USB Type-C pin assigment D got enabled
> and USB3.0 devices were working in parallel to video ouput.
>
> Known issues:
> * As mentioned above, it appears that on Gnome+Wayland framerate and
> resolution parameter adjustment is not stable.
>
> Due to lack of access to the official DisplayPort specfication, changes
> were primarily inspired by/reverse engineered from Intel's i915 driver.
>
> [1] https://lore.kernel.org/all/20250226231436.16138-2-alex.vinarskis@gmail.com/
>
> Signed-off-by: Aleksandrs Vinarskis <alex.vinarskis@gmail.com>
>
> ---
>
> 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/
>
> Changes in v2:
> - Picked up Abel's R-b tags
> - Fixed typo as per Abel, fixed readability as per Johan
> - Updated cover and commit message on mailink issue which appears to be
> specific to Gnome+Wayland. No problems on Xorg.
> - Link to v1: https://lore.kernel.org/all/20250310211039.29843-1-alex.vinarskis@gmail.com/
>
> 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
>
> drivers/gpu/drm/msm/dp/dp_ctrl.c | 126 ++++++++++++++++++++--------
> drivers/gpu/drm/msm/dp/dp_display.c | 27 ++++--
> drivers/gpu/drm/msm/dp/dp_link.h | 4 +
> drivers/gpu/drm/msm/dp/dp_panel.c | 12 ++-
> 4 files changed, 122 insertions(+), 47 deletions(-)
>
> --
> 2.45.2
>