[Bug 89761] New: [i915/HD5500/SNA] Switching from X session to VT causes unrecoverable corruption/drift.

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=89761

            Bug ID: 89761
           Summary: [i915/HD5500/SNA] Switching from X session to VT
                    causes unrecoverable corruption/drift.
           Product: xorg
           Version: unspecified
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/intel
          Assignee: chris at chris-wilson.co.uk
          Reporter: login at rmup.net
        QA Contact: intel-gfx-bugs at lists.freedesktop.org

Hardware: ThinkPad T550 with i7-5600U/HD5500/i915/3K display
Versions: xf86-video-intel 2.99.917, using SNA
          xorg-server 1.17.1
          linux 3.19/4.0rc5

When killing the active X session and returning to the VT, or when changing
from an X session to a different VT, the display starts to drift and flicker:
https://youtu.be/Mez3XlMkK-o . The display goes completely black eventually,
with small flickers of data here and there when trying to change VT for
example.

Can be reproduced with xf86-video-intel > 2.99.914. There seems to be some
difference with what kernel is used. On 3.19 the display breaks most of the
time when killing the X session, but not when changing from a X session to
another VT. On 4.0rc5 the display always breaks both when switching from a X
session to another VT and when killing the X session.

After the display drift has started, starting a new X session, trying to switch
back to existing X session, etc, does not help recovering from the problem. In
addition to a reboot, suspending and resuming seems to stop it.

The problem is not present when using UXA, or when using modesetting driver
instead of intel. Arch Linux was used for testing, with matching xorg-server
version (1.17.1 for >2.99.914, 1.16.0 when testing with 2.99.914).

Reverting git commit 57bf3d ("sna: Disable the fb on switching away from X")
seems to solve the issue.

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