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

Aleksandrs Vinarskis
2025-04-17T02:13:53Z

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

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(-)

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2.45.2