====== Add display modelines ======

If a resolution isn't available in X while the monitor should support it, the modeline can be added manually.

First get the port names of the connected monitors:
<code>
xrandr
Screen 0: minimum 320 x 200, current 4080 x 1956, maximum 16384 x 16384
DP-1 connected primary 1920x1200+1080+432 (normal left inverted right x axis y axis) 518mm x 324mm
   1920x1200     59.95*+
   1920x1080     60.00  
   1600x1200     60.00  
   1680x1050     59.88  
   1280x1024     60.02  
   1280x960      60.00  
   1024x768      60.00  
   800x600       60.32  
   640x480       59.94  
   720x400       70.08  
DP-2 disconnected (normal left inverted right x axis y axis)
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-2-8 connected 1080x1920+3000+36 left (normal left inverted right x axis y axis) 160mm x 90mm
   1920x1080     60.00*+  50.00    59.94  
   1600x1200     60.00  
   1680x1050     59.88  
   1280x1024     60.02  
   1280x960      60.00  
   1280x720      60.00    50.00    59.94  
   1024x768      60.00  
   800x600       60.32  
   720x480       60.00    59.94  
   640x480       60.00    59.94  
   720x400       70.08  
DP-2-9 connected 1080x1920+0+0 left (normal left inverted right x axis y axis) 160mm x 90mm
   1920x1080     60.00*+  50.00    59.94  
   1280x720      60.00    50.00    59.94  
   720x480       60.00    59.94  
   640x480       60.00    59.94  
</code>
Here, monitors are connected on DP-1, DP-2-8 and DP-2-9.

Calculate modeline for the resolution required:
<code>cvt 1920 1200
# 1920x1200 59.88 Hz (CVT 2.30MA) hsync: 74.56 kHz; pclk: 193.25 MHz
Modeline "1920x1200_60.00"  193.25  1920 2056 2256 2592  1200 1203 1209 1245 -hsync +vsync
</code>
It probably creates a 60Hz modeline, if other refresh rates are required, add the refresh rate:
<code>
cvt 1920 1200 50
# 1920x1200 49.93 Hz (CVT 2.30MA) hsync: 61.82 kHz; pclk: 158.25 MHz
Modeline "1920x1200_50.00"  158.25  1920 2040 2240 2560  1200 1203 1209 1238 -hsync +vsync
</code>

Now add the relevant modes to xrandr:
<code>
xrandr --newmode "1920x1200_60.00"  193.25  1920 2056 2256 2592  1200 1203 1209 1245 -hsync +vsync
xrandr --newmode "1920x1200_50.00" 158.25  1920 2040 2240 2560  1200 1203 1209 1238 -hsync +vsync
</code>

Lastly, add the mode to the relevant monitor:
<code>
xrandr --addmode DP-2-9 1920x1200_60.00
xrandr --addmode DP-2-8 1920x1200_60.00

xrandr --addmode DP-2-9 1920x1200_50.00
xrandr --addmode DP-2-8 1920x1200_50.00
</code>

Switch to the modes and test.

Note that this is not permanent and after a reboot, the modelines do not exist any longer.

Add to rc.local / .profile or whatever best applies to your system.

This can be applied in lightdm:
<code - /etc/lightdm/lightdm.conf.d/70-linuxmint.conf>
[SeatDefaults]
user-session=cinnamon

[Seat:*]
display-setup-script=xrandr --newmode "1920x1200_50.00" 158.25  1920 2040 2240 2560  1200 1203 1209 1238 -hsync +vsync ;\
                     xrandr --addmode DP-2-9 1920x1200_50.00 ;\
                     xrandr --addmode DP-2-8 1920x1200_50.00 ;\
                     xrandr --output DP-1 --primary --mode 1920x1200 --pos 1200x432 --rotate normal --output DP-2 --off --output HDMI-1 --off --output DP-2-8 --mode 1920x1200_50.00 --pos 3120x0 --rotate left --output DP-2-9 --mode 1920x1200_50.00 --pos 0x0 --rotate left
</code>