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

[Lyude Paul]

hi - I almost certainly know the solution to this, the patches that we got
from amd to do bandwidth checking in the DP MST helpers don't actually work
correctly in a lot of cases and I need to fix them. I've just been busy on PTO
and only just got back today, and have been busy with fixing a lot of RHEL
stuff at the same time. I'll take a closer look at this soonb

On Wed, 2020-02-26 at 16:15 +0100, Hans de Goede wrote:
> 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
> 
-- 
Cheers,
	Lyude Paul (she/her)
	Associate Software Engineer at Red Hat