[Bug 94578] New: Unplugging external monitor reliably wedges X on thinkpad t450s

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=94578

            Bug ID: 94578
           Summary: Unplugging external monitor reliably wedges X on
                    thinkpad t450s
           Product: DRI
           Version: unspecified
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: major
          Priority: medium
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: njs at pobox.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

Created attachment 122353
  --> https://bugs.freedesktop.org/attachment.cgi?id=122353&action=edit
intel_reg dump --all output, before plugging external monitor in

On a thinkpad t450s (broadwell), I can plug in an external monitor and it works
great. But when I unplug it, then the most common outcome is that I drop into a
weird crash state:

- I get dropped to a text console on vt1, which functions fine -- I can log in,
collect debugging information, etc.
- the X server itself is still running -- e.g. I can do "DISPLAY=:0 xrandr" and
it happily reports back that the external monitor is disconnected and that the
built-in panel is configured and running.
- applications connected to the X server in general are still running (e.g.
they show up on 'ps', clementine continues to play music out the speakers,
etc.)
  - exception: there's a line in dmesg saying "traps: gnome-shell[1172] trap
int3 ip:7f31e41a887b sp:7ffe72d29a70 error:0", so I guess gnome-shell has
probably crashed?
- aside from this line about gnome-shell, there's nothing at all reported in
dmesg -- no BUG or backtrace or anything like that
- there's nothing at all reported in Xorg.0.log
- if I then press alt-f2 to switch back to vt2, where the X server is running,
then the screen goes blank and input locks up.
  - except, if I press control-alt-del to reboot, then systemd successfully
sees the control-alt-del, and successfully prints some stuff on the screen, so
I guess it's really showing vt2 in text mode?

This is using Debian testing's userspace (xserver-xorg-core 1.18.1,
xserver-xorg-video-intel 2.99.917+git20160307-2), and I get the same behavior
with both Debian's default 4.3 kernel and with a 4.5.0rc7 from Debian
experimental.

~$ uname -a
Linux branna 4.5.0-rc7-amd64 #1 SMP Debian 4.5~rc7-1~exp1 (2016-03-07) x86_64
GNU/Linux

The external monitor is an HDMI cable attached to an HDMI-to-miniDP dongle,
plugged into the miniDP port on the laptop.

This is extremely frustrating (basically makes my big work monitor unusable)
and extremely reproducible, so if you have any debugging suggestions I'm very
motivated to try :-). (Difficulty level: I've written (small) kernel and Xorg
patches before; I'd be debugging it myself already if I had any idea where to
start :-).)

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