I run Ubuntu 18 on a PC connected to a 3440x1440 display. It is connected with a DisplayPort to HDMI adapter and a modern HDMI cable. The highest resolution available in the display settings is 1920x1080.
I tried using xrandr to force it (at both 60 and 50 hz).
$ xrandr --newmode "3440x1440_50.00" 344.00 3440 3672 4040 4640 1440 1443 1453 1484 -hsync +vsync $ xrandr --addmode HDMI-1 3440x1440_50.00 $ xrandr --output HDMI-1 --crtc 0 --mode 3440x1440_50.00
But I get an error.
xrandr: Configure crtc 0 failed
I know that the adapter and cable can support a signal at this resolution, because if I plug the adapter into my Windows 10 machine, it uses the native resolution automatically.
I have also observed that the Ubuntu machine can output 3440x1440 over DisplayPort automatically. I would use the DisplayPort output, but unfortunately the monitor only has one DisplayPort input. I would swap the connections since the HDMI signal works from Windows, but it does so at a lower framerate, which doesn't work for my purposes.