[Bug 99128] New: External monitor ("reverse PRIME", Output Sink) updates very slowly (1fps) when laptop screen is off

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=99128

            Bug ID: 99128
           Summary: External monitor ("reverse PRIME", Output Sink)
                    updates very slowly (1fps) when laptop screen is off
           Product: xorg
           Version: git
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/intel
          Assignee: chris at chris-wilson.co.uk
          Reporter: peter at lekensteyn.nl
        QA Contact: intel-gfx-bugs at lists.freedesktop.org

Scenario:
Hybrid graphics laptop with an laptop screen (eDP-1, Intel) and an external
monitor connected to the Nvidia GPU (DP-1, nouveau).
DP-1 (nouveau) is linked as output slave to the Intel GPU.
(When Intel renders a picture, it is shown on the external screen connected to
the Nvidia GPU.)

Problem:
When the lid is closed (and no outputs are active for Intel), stuff on the
external monitor updates only every one second.

Steps to reproduce:
1. Enable output source:

    xrandr --setprovideroutputsource nouveau Intel
    xrandr --output DP-1 --mode 2560x1440 --left-of eDP-1
2. Run "glxgears" and drag it to the external monitor.
3. Turn off Intel screen (e.g. xrandr --output eDP-1 --off or by closing the
lid).

Expected behavior:
glxgears should be updating normally with 60fps.

Actual behavior:
glxgears updates with 1fps.

Other information:
A similar problem existed in the modesetting driver, fixed in Xorg 1.19 with
https://lists.x.org/archives/xorg-devel/2016-August/050797.html
I think that a similar fix can be developed for sna_display.c
(sna_covering_crtc).

Environment:
Arch Linux x86_64
Intel i7-6700HQ, Nvidia GT 965M
xorg-server 1.19.0-3 (but it occurred with 1.18.4 too)
xf86-video-intel 1:2.99.917+746+g169c74f-2
xf86-video-nouveau 1.0.13+7+g1516d35-2

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