[Bug 89658] New: Displays on T440s' external DP over MST regularly blank

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=89658

            Bug ID: 89658
           Summary: Displays on T440s' external DP over MST regularly
                    blank
           Product: xorg
           Version: unspecified
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/intel
          Assignee: chris at chris-wilson.co.uk
          Reporter: tim.besard at gmail.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: airlied at freedesktop.org, tim.besard at gmail.com

I'm using Arch Linux (shipping xf86-video-intel 2.99.917 and xorg-server
1.17.1)
on my T440s with two external Dell 24" monitors connected over both DVI and DP
to a docking station. This specific docking station (official Ultra dock) uses
DP MST to wire the external connections.

When connected to the external displays, I'm not using my laptops screen (lid
closed). I'm configuring my external screens as follows:

$ xrandr --output DP2-2 --auto --output DP2-1 --auto --right-of DP2-2

Once in a while, one of the external monitors "desynchronizes" and blanks (in a
very peculiar fashion, detailed below), only to restore its image quickly
afterwards. This happens on both of my external displays (never on the laptop
screen), about once every 15 minutes (sometimes more frequently, sometimes
less).

A video of the blanking:
http://users.elis.ugent.be/~tbesard/public/mst-desync.mp4

This is a 4x slowed down recording of my screen blanking. Note that just before
my screen blanks, it kinda "desynchronizes" (clearly visible at 3.4s). In some
cases, this phenomena is way stronger, eg. straight lines go all wobbly for a
full second, then seem to go back to normal for a while, but the screen still
blanks quickly after that.

Also interesting is how the screen blanks, it *always* happens in this two-step
fashion as seen in the video: ~.25s blank, ~.5s normal again, ~2s blank. Very
occasionally (once a week) the screen in question remains dead after all this.

I have been tracking Dave Airlie's MST patches since they first arrived on his
3.14 branch kernel on fdo, and I distinctly remember this bug not being there
in
the first weeks on being able to drive my external displays. However, I have
not
been able to go back to that stable situation, despite trying all of the old
branches (starting at drm-i915-mst-v3.14) and patchsets. I haven't tried to
revert my xorg driver though (because of it being relatively hard due to ABI
dependencies).

Sadly, I cannot provide much more information, no additional messages are
generated in dmesg (booting with drm.debug=0x06). I've posted some more
information (register snapshots, opregions) in bug #89612, which is otherwise
unrelated but happens to be on the exact same hardware.

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