5.6 DP-MST regression: 1 of 2 monitors on TB3 (DP-MST) dock no longer light up

[Hans de Goede]

Hi Lyude and everyone else,

Lyude I'm mailing you about this because you have done a lot of
work on DP MST, but if this rings a bell to anyone else feel
free to weigh in on this.

I'm currently using a Lenovo X1 7th gen + a Lenovo TB3 gen 2 dock
as my daily rider for testing purposes. When 5.6-rc1 came out I
noticed that only 1 of the 2 1920x1080 at 60 monitors on the dock
lights up.

There are no kernel errors in the logs, but mutter/gnome-shell says:

gnome-shell[1316]: Failed to post KMS update: Page flip of 93 failed

With 93 being the crtc-id of the crtc used for the monitor which is
displaying black. Since then I've waited for 5.6-rc3 hoping that a
fix was already queued up, but 5.6-rc3 still has this problem.

gnome-shell does behave as if all monitors are connected, so the
monitor is seen, but we are failing to actually send any frames
to it.

I've put a log collected with drm.debug=0x104 here:
https://fedorapeople.org/~jwrdegoede/drm-debug.log

This message stands out as pointing to the likely cause of this problem:

[    3.309061] [drm:intel_dump_pipe_config [i915]] MST master transcoder: <invalid>

Regards,

Hans