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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
  • I seem to have the same issue, just installed Ubuntu 20.04 on a Dell Precision 5540. I could get the full monitor resolution in Windows but on Linux I'm capped at 3440x1440. Did you make any progress, file any bug report somewhere that I can follow? – ajn Apr 24 at 6:03
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I too have the LG 34WK95U monitor. I was able to get my Dell XPS 7590 (OLED) working with an external 4k monitor (HDMI at 30Hz), and the LG at 30Hz via USB-C.

For reference, this is on Ubuntu 20.04.

While the refresh rate certainly isn't ideal, at least the LG resolution is correct now.

Here's what I did:

$ cvt 5120 2160 30 
# 5120x2160 29.98 Hz (CVT) hsync: 65.96 kHz; pclk: 452.75 MHz

$ xrandr --newmode "5120x2160_30.00"  452.75  5120 5448 5992 6864  2160 2163 2173 2200 -hsync +vsync

$ xrandr --addmode DP-1 "5120x2160_30.00"
$ xrandr --output DP-1 --mode "5120x2160_30.00"

After running the last command, I did have some overlap of another desktop in one corner. Opening the Settings app and adjusting the monitor layout resolved this.

And the full xrandr output:

$ ~ xrandr
Screen 0: minimum 320 x 200, current 8960 x 4320, maximum 16384 x 16384
eDP-1 connected primary 3840x2160+1689+2160 (normal left inverted right x axis y axis) 344mm x 194mm
   3840x2160     60.00*+  59.98    59.97  
   3200x1800     59.96    59.94  
   2880x1620     59.96    59.97  
   2560x1600     59.99    59.97  
   2560x1440     59.99    59.99    59.96    59.95  
   2048x1536     60.00  
   1920x1440     60.00  
   1856x1392     60.01  
   1792x1344     60.01  
   2048x1152     59.99    59.98    59.90    59.91  
   1920x1200     59.88    59.95  
   1920x1080     60.01    59.97    59.96    59.93  
   1600x1200     60.00  
   1680x1050     59.95    59.88  
   1600x1024     60.17  
   1400x1050     59.98  
   1600x900      59.99    59.94    59.95    59.82  
   1280x1024     60.02  
   1440x900      59.89  
   1400x900      59.96    59.88  
   1280x960      60.00  
   1440x810      60.00    59.97  
   1368x768      59.88    59.85  
   1360x768      59.80    59.96  
   1280x800      59.99    59.97    59.81    59.91  
   1152x864      60.00  
   1280x720      60.00    59.99    59.86    59.74  
   1024x768      60.04    60.00  
   960x720       60.00  
   928x696       60.05  
   896x672       60.01  
   1024x576      59.95    59.96    59.90    59.82  
   960x600       59.93    60.00  
   960x540       59.96    59.99    59.63    59.82  
   800x600       60.00    60.32    56.25  
   840x525       60.01    59.88  
   864x486       59.92    59.57  
   800x512       60.17  
   700x525       59.98  
   800x450       59.95    59.82  
   640x512       60.02  
   720x450       59.89  
   700x450       59.96    59.88  
   640x480       60.00    59.94  
   720x405       59.51    58.99  
   684x384       59.88    59.85  
   680x384       59.80    59.96  
   640x400       59.88    59.98  
   576x432       60.06  
   640x360       59.86    59.83    59.84    59.32  
   512x384       60.00  
   512x288       60.00    59.92  
   480x270       59.63    59.82  
   400x300       60.32    56.34  
   432x243       59.92    59.57  
   320x240       60.05  
   360x202       59.51    59.13  
   320x180       59.84    59.32  
DP-1 connected 5120x2160+3840+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    59.94  
   1024x768      60.00  
   800x600       60.32  
   720x480       60.00    59.94  
   640x480       60.00    59.94  
   5120x2160     59.99  
   5120x2160_30.00  29.98* 
DP-2 connected (normal left inverted right x axis y axis)
   2560x2160     59.66  
   640x480       59.94  
DP-3 connected 3840x2160+0+0 (normal left inverted right x axis y axis) 697mm x 392mm
   3840x2160     30.00*   25.00    24.00    29.97    23.98  
   2560x1440     59.95  
   2048x1280     59.99  
   2048x1080     24.00  
   1920x1080     60.00    60.00    50.00    59.94    25.00    24.00    23.98  
   1920x1080i    60.00    50.00    59.94    50.00  
   1600x1200     60.00  
   1600x900      60.00  
   1280x1024     75.02    60.02  
   1152x864      75.00  
   1280x720      60.00    50.00    59.94  
   1024x768      75.03    60.00  
   800x600       75.00    60.32  
   720x576       50.00  
   720x480       60.00    59.94  
   640x480       75.00    60.00    59.94  

Ideally this should be supported out of the box. But the 5120x2160 resolution isn't recognized by default in Gnome Settings. Hopefully this information helps someone else!

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