[Bug 108907] Display corruption on multi-monitor when the largest panel is not on the left (and amdgpu.dc=1)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=108907

            Bug ID: 108907
           Summary: Display corruption on multi-monitor when the largest
                    panel is not on the left (and amdgpu.dc=1)
           Product: DRI
           Version: unspecified
          Hardware: x86-64 (AMD64)
               URL: https://imgur.com/a/DIqAfu1
                OS: Linux (All)
            Status: NEW
          Severity: major
          Priority: medium
         Component: DRM/AMDgpu
          Assignee: dri-devel at lists.freedesktop.org
          Reporter: kdebugs at funkster.org

After doing an update (Arch) on 2018-11-27 (including mesa (18.2.4-1 ->
18.2.5-1), linux (4.19.2.arch1-1 -> 4.19.4.arch1-1)) I found I had corruption
on the center monitor of a PLP setup. Further investigation found that the
corruption does not occur if the large panel is configured to be on the left
(both using XFCE display settings, and the equivalent in KDE). This was
replicated on both my installed OS and a Manjaro 18.0 live USB.

Corruption takes the form of every other line being offset by an increasing
amount, although it only starts part-way down the monitor. Side monitors are
not affected.

https://i.imgur.com/0pQMznF.jpg

Probably important details:
- Arch / XFCE / XDE / lightdm
- AMDGPU open source drivers (Sapphire Radeon RX580)
- 3 monitors in PLP: sidecars are 1680x1050 (but turned so they're vertical),
center is 2560x1600. Left-hand sidecar is on HDMI, other two are DP.
- Corruption DOESN'T happen if the large monitor is placed on the left! Nor
does it happen at the login screen (which doesn't have any screen rotation and
probably also has the big panel on the left)
- Corruption DOESN'T happen if I reduce the resolution of the center screen to
2048x1280 (but obviously that's not a solution)
- Corruption DOESN'T happen if I pass amdgpu.dc=0 to the kernel
- Corruption DOES happen on KDE (tested with Manjaro KDE live USB) as long as
you arrange the monitors in the same way

Here's a paste of the pacman.log from the day it all went wrong:
https://pastebin.com/H3i5LuzU
or
https://hastebin.com/dixemisepo.php

Thank you for your attention.

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