[Bug 106529] Kernels 4.16+ confuses KDE wrt what the primary display is when waking up multiple displays

[bugzilla-daemon@freedesktop.org]

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https://bugs.freedesktop.org/show_bug.cgi?id=106529

            Bug ID: 106529
           Summary: Kernels 4.16+ confuses KDE wrt what the primary
                    display is when waking up multiple displays
           Product: DRI
           Version: unspecified
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/AMDgpu
          Assignee: dri-devel@lists.freedesktop.org
          Reporter: mariusz.g.mazur@gmail.com

Created attachment 139577
  --> https://bugs.freedesktop.org/attachment.cgi?id=139577&action=edit
ubuntu's 4.15 dmesg with amdgpu.dc_log=1

It appears that the display-handling code in 4.15+ (including 4.17-rc5) marks
any display not fully woken up as disconnected or something to that effect. So
when I have multiple monitors and they get woken up from sleep (doesn't matter
whether the computer was suspended or just idle for a long period), and my
primary display wakes up slower than the any of the other displays (and it
does), then for a few hundred ms KDE tries to change the primary display to the
one that first woke up (I'm guessing it thinks that's the only one available),
then changes its mind a bit later, when all displays wake up.

End result looks like crap and is not usable, since all my windows get
displaced. Here's how it looks like in practice:

Setup:
Radeon RX 560
Primary display: bottom right, displayport, on
Secondary display 1: left, hdmi, on
Secondary display 2: top right, dvi, off

Video of how it used to work, using 4.4.15:
https://www.youtube.com/watch?v=h7nMYbm5ZxU
Note how even though the left (secondary) display wakes up first, both
instantly display their proper desktops.

Video of how it works with ubuntu 18.04's default 4.15 kernel/4.16.7/4.17-rc5:
https://www.youtube.com/watch?v=VdVFNXPszSI
Note how when the left (secondary) display wakes up first, KDE decides that
this is probably the only display available, so tries to switch the primary
desktop to it, then a split second later the bottom right wakes up and KDE
knows *that* should be the primary according to the config, so it switches
back.

Attached is dmesg from ubuntu's 4.15 with dc_log=1. I put the computer to
sleep, then wake it up and get the behavior like in the video.

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