[Bug 201795] [Regression] Wrong 4k resolution detected with DisplayPort to HDMI adapter on amdgpu

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=201795

--- Comment #22 from thomas.lassdiesonnerein at gmx.de ---
I investigated further and the culprit seems to be that kernel =4.14 ignores
what xrandr *wrongly* detects and sets the connected TV correctly to its native
3840x2160 at 60hz without black bars. Xrandr detects only 30hz in both 4k
resolutions on all kernels. Kernel 4.14 still manages somehow to give me
3840x2160 at 60hz without black bars. So could this be a xrandr-bug?

Kernel >4.14 seems to respect xrandr and sets: 4096x2160 at 30hz without black
bars - maybe my HDMI2.0-to-DP adapter is reporting this setting.
I also can get 3840x2160 at 30hz without black bars with your xrandr workaround,
but not 60hz.


In more detail:

kernel =4.14
-------------
xrandr --props reports that scale-mode is "None". But still no black bars,
because the *native* resolution of the TV is 3840x2160 (listed in the panel
specs here: https://www.lg.com/us/support-product/lg-OLED65B7P)

So if I change to 4096er resolution the desktop is bigger than the screen.
Also I have 60hz on both displays, while xrandr *wrongly* reports only 30hz
available on the TV. The difference between 60 and 30hz is very visible when
moving the mouse and e.g. supertuxkart confirms 60hz in the FPS-overlay.


kernel >4.14
--------------
The TV sets only 30hz, on 4096 and 3840 res. When I force a 60hz-modeline via
xorg.conf.d like described here
https://wiki.archlinux.org/index.php/xrandr#Adding_undetected_resolutions I get
no signal or a switch back to 30hz, when cloning in the GUI.

Then the 3840-black-bar workaround only works if I create this file: 

cat /etc/xdg/autostart/Scaling.desktop

[Desktop Entry]
Type=Application
Name=Scaling
Exec=xrandr --output DisplayPort-0  --set "scaling mode" "Full"


Putting it in xorg.conf had no effect. This is what my xorg.conf now looks
like. Note that the VRR Option works as expected, so its not as if the file is
completely ignored:


cat /etc/X11/xorg.conf.d/10-monitor.conf 

Section "Monitor"
    Identifier "DisplayPort-0"
    Option "scaling mode" "Full" 
    Option "PreferredMode" "3840x2160"
EndSection

Section "Screen"
   Identifier "Screen0"
  Device "AMD"
EndSection

Section "Device"
    Identifier "AMD"
    Driver "amdgpu"
  Option "DRI"         "3"
    Option "TearFree"    "true"
   Option "VariableRefresh" "true"
EndSection

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