[PATCH v2 0/2] drm/msm/dp: Introduce link training per-segment for LTTPRs

[Stefan Schmidt]

Hello Aleksandrs,

On Wed, 12 Mar 2025 at 00:41, Aleksandrs Vinarskis
<alex.vinarskis at 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
>         * 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
>         * 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)

You can  add the following:
* Dell XPS 9345, Debian trixie/sid, Gnome 48, Wayland
        * 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 at 100hz and 4k at 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.

I can confirm this on Gnome 48 + Wayland as well. Sometimes the resolution
change from gnome settings gets stuck and does not apply. It normally works
here around every third try or so when using a dock.

> 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 at gmail.com>

Tested-by: Stefan Schmidt <stefan.schmidt at linaro.org>

regards
Stefan Schmidt