[Bug 90083] New: unusable desktop with dell u2515h

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=90083

            Bug ID: 90083
           Summary: unusable desktop with dell u2515h
           Product: xorg
           Version: git
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: major
          Priority: medium
         Component: Driver/Radeon
          Assignee: xorg-driver-ati at lists.x.org
          Reporter: dev.frandom at gmail.com
        QA Contact: xorg-team at lists.x.org

Created attachment 115171
  --> https://bugs.freedesktop.org/attachment.cgi?id=115171&action=edit
xorg.log loop monitor modes

Hello,

first things first: i do not know whether this behavior is caused by the radeon
driver or the xorg server as i was not able to reproduce it with a wayland live
cd. But as the ati-drivers did not show the same symptoms my best guess is the
radeon driver.

The situation: 
Recently i bought a new monitor (dell u2515h) for my system (gentoo (mostly
stable), amd fx 8350, amd hd 7750) as a replacement for my previous one (dell
u2412m). Everything (boot, sddm) went well until i started the plasma 5
desktop. The login splash' progress bar reached around 70% but after that no
progress at all. Starting an empty xsession with xterm only is possible, but
after launching the kwin window manager, the reason showed up, why the plasma
desktop didn't want to start successfully: the mouse movement stuttered and was
horrible slow. The xorg.log (see attachment) shows that something causes the
xorg server to print the monitor modes over and over again, the only way to
stop it is killing the xorg server.

Switching to the xfce desktop helped temporarily, but it is very disappointing,
as i got the same stuttering after starting the desktop. Just after the 5th or
6th try (sometimes more, sometimes less) the effect does not show up and i get
a usable desktop.

So i tried to reproduce this situation in a plain xsession with xterm only.
Setting the dpi via xrandr to 118 was the easiest way to raise the mouse
stuttering. To make things more interestingly, very rarely the command was
successful. Then i even was able to start kwin and some kde applications, but
as soon as i started plasma the mouse stuttering was back.

I have testet different linux distributions, they all behave the same.
Xorg-Server (currently 1.16.4, temporarily 1.17.1) does not change a thing.
Using the ati-driver is a solution to this problem, but not suitable for me
(and possibly for many others).

I'll provide any information (which i'm capable of) you need to resolve this
issue.

Thanks!

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