Add not detected resolutions on Xorg

Supposing you are using lightdm and your monitor has support to 2560×1080@75.00. If your monitor is identified by HDMI2.

First, execute:

cvt 2560 1080 75

The output will be something like that:

Modeline “2560x1080_75.00” 294.00 2560 2744 3016 3472 1080 1083 1093 1130 -hsync +vsync

Now, copy this line without “Modeline” part. Paste this string into the following script file: /root/addmodeset.sh. The content of this file will be:

#!/bin/sh

xrandr –newmode “2560x1080_75.00” 294.00 2560 2744 3016 3472 1080 1083 1093 1130 -hsync +vsync
xrandr –addmode HDMI2 2560x1080_75.00

Now, add the following line to file /etc/lightdm/lightdm.conf:

display-setup-script=/root/addmodeset.sh

Finally, make script executable:

sudo chmod +x /root/addmodeset.sh

Reboot the system and add the resolution via system preference.