[Bug 97375] [BSW]Long time test display, display exception, and got *ERROR* CPU pipe x FIFO underrun error.

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=97375

--- Comment #12 from M. G. <gentoo at gishpuppy.com> ---
Created attachment 132020
  --> https://bugs.freedesktop.org/attachment.cgi?id=132020&action=edit
drm-tip dmesg with drm.debug=0xe

The bug is still present in drm-tip commit
028a0b3762b318a72ab04675355db97c854c230b (from 2017-05-25 - not 2017-05-22,
sorry). Please see attached dmesg log. Please also note the kernel warning
including the trace regarding a backlight issue. Or is this a subsequent error
due to the underrun?

Here is what I did when the bug was triggered:
- I did a Suspend to RAM while no external monitor was connected
- I connected an external monitor via HDMI (plus keyboard and mouse via USB)
- I opened my laptop to resume from suspend. Output on internal LCD screen was
OK and I executed a script in order to switch output from internal LCD to
external screen. The script basically does:

 xrandr --output eDP1 --off
 xrandr --output HDMI1 --auto --left-of eDP1 --dpi 96x96

I got only a black screen on the external monitor (backlight on). To solve the
problem I executed another script using a hotkey, which does:

 xrandr --output HDMI1 --off
 xrandr --output eDP1 --auto --primary --dpi 96x96

Then I executed my first script again and output on external monitor was OK.

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