[Bug 108934] [Dualscreen] "No signal" issues when booting with DVI and HDMI screens both plugged in

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=108934

            Bug ID: 108934
           Summary: [Dualscreen] "No signal" issues when booting with DVI
                    and HDMI screens both plugged in
           Product: DRI
           Version: unspecified
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/AMDgpu
          Assignee: dri-devel at lists.freedesktop.org
          Reporter: laurent.pointecouteau at gmail.com

I'm using a AMD R9 380 video card on Arch Linux (with kernel 4.19.4 and the
latest `mesa` package installed), and since several weeks, dualscreen has a
buggy behavior. My Samsung TV displays "no signal" after booting while plugged
in my computer (using a HDMI cable), and when trying to switch display modes
using the GNOME control center (or the Super+P shortcut), my Iiyama ProLite
B2475HDS DVI monitor gets randomly disabled and displays "no signal" too
without ever recovering, forcing me to hard reboot.

I've tried to apply a kernel patch as suggested in bug #108704, but it only
removed the amdgpu-related error messages from dmesg, without fixing the
dualscreen issues. Since then, I've found two workarounds:
 - putting the system to sleep then waking it up seems to restore normal
functionality;
 - when booting with the HDMI cable unplugged, after plugging it in, screen
switching is not buggy anymore.

My setup does not allow me to safely downgrade to an older kernel, but I can
confirm that the screen settings started to exhibit this buggy behaviour only
recently.

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