I bought a new LG 34WK95U ultra-wide screen, which has a native resolution of 5120x2160 at 60Hz.

When connecting the monitor via a Thunderbolt 3 USB-C connection, it gets picked up, but it fails to probe for the correct resolution. Also, once connected, I now get two (!) new outputs. Here is my current xrandr:

% xrandr

Screen 0: minimum 320 x 200, current 3440 x 1440, maximum 8192 x 8192
eDP-1 connected (normal left inverted right x axis y axis)
   # Snip; removed the laptop modes  
DP-1 connected (normal left inverted right x axis y axis)
   2560x2160     59.66  
   640x480       59.94  
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-2 connected primary 3440x1440+0+0 (normal left inverted right x axis y axis) 800mm x 330mm
   3440x1440     59.97*+  59.96    49.99  
   3840x2160     60.00    30.00  
   1920x1080     60.00    60.00    59.94  
   1600x900      60.00  
   1280x1024     60.02  
   1280x800      59.81  
   1152x864      59.97  
   1280x720      60.00    60.00    59.94  
   1024x768      60.00  
   800x600       60.32  
   720x480       60.00    59.94  
   640x480       60.00    59.94    59.94  
   5120x2160_60.00  59.99  
   5120x2160_30.00  29.98  
HDMI-2 disconnected (normal left inverted right x axis y axis)

Those DP-1 and DP-2 popped up after I connected the USB-C to the external screen. I am not sure what the meaning of the second output is. Interestingly, when I did this the first time around, DP-1 had all the modes listed, and DP-2 only had those two right now under DP-1.

You can also see the additional modes I added, 5120x2160_60.00 and 5120x2160_30.00 by running % cvt 5120 2160 30 and 60 followed by xrandr --newmode and xrandr --addmode, respectively.

The result of calling xrandr with either of these two modes gives this:

% xrandr --output DP-2 --mode 5120x2160_30.00                                                    
xrandr: Configure crtc 0 failed
  • Guess: USB C alternate mode is not working properly (after all, support is quite new). What computer and what graphic card are you using? At least look into /var/log/Xorg.0.log to find out a bit more what the X server thinks it's doing. But very likely you'll have to file a bug with the USB C alternate mode developers (and if this is on a Mac, getting that to work will be difficult). – dirkt Dec 6 '18 at 15:28

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