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

Dmitry Baryshkov
2025-04-24T22:14:26Z

On Thu, Apr 24, 2025 at 02:12:15PM -0700, Rob Clark wrote:
> 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

Could you please send this as a response to one of the patches, so that
it's also recorded by the patchwork?

>
> 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
> >

--
With best wishes
Dmitry