[Bug 110863] New: Blank screen after locking and vt-switch

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=110863

            Bug ID: 110863
           Summary: Blank screen after locking and vt-switch
           Product: DRI
           Version: unspecified
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: corsac at debian.org
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

Hi,

I'm reporting a bug as the Debian maintainer of the Xfce desktop environment.
Since people have started to upgrade to future new stable version (Buster,
currently testing) I have had multiple bug reports with a similar behavior.

The locking infrastructure in Debian buster with Xfce relies on following
components:

- LightDM (the login screen manager) with its greeter, lightdm-gtk-greeter
- light-locker: the lock screen

light-locker works by drawing a simple window on top of the desktop on VT7 and
locking it, then doing a vt-switch to a new lightdm on VT8. When entering the
correct login/password on the login screen, a DBus message is sent to
light-locker which unlocks the screen.

Unfortunately, on setups with Intel adapters (of various generations, I
personnaly reproduced on a ThinkPad X250 with Broadwell and Thinkpad X230 with
Ivy Bridge) when the screen is then shut down (for power saving or because of a
suspend), the screen doesn't turn back on when moving the mouse or entering
keys.

As far as I can tell the only way to bring back the screen is to manually
switch VT (with ctrl-alt-Fx).

There's a downtream bug report at
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=805711 but unfortunately
there's a lot of noise so I'm a bit lost myself.

There's an upstream bug report on Light Locker
(https://github.com/the-cavalry/light-locker/issues/108) but it doesn't seem
completely related.


uname -m: x86_64 (but I guess i386 is affected as well)
uname -r: 4.19.0-4-amd64
Linux Distribution: Debian testing/sid
Machine: at least ThinkPads X230 and X250 for me but Debian bug report shows a
lot more
Display connector: LVDS, eDP, DP (and I guess HDMI, VGA...)
- DDX is modesetting from Xorg core

If you want more specific information please ask. I know the issue is weird.
It's likely a bad interaction between various components (i915, DDX) and a
not-often used codepath (vt-switch + DPMS maybe?)

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