Can't set screen scaling modes for external monitors.
Submitted by Madper Xie
Assigned to Intel GFX Bugs mailing list
Currently, it is possible to set scaling mode for a monitor only if the monitor is internal (LVDS, eDP). This is done through xrandr:
$ xrandr --prop
Screen 0: minimum 320 x 200, current 1680 x 1050, maximum 8192 x 8192
LVDS1 connected primary 1680x1050+0+0 (normal left inverted right x axis y axis) 331mm x 207mm
scaling mode: Full aspect
supported: None, Full, Center, Full aspect
$ xrandr --output LVDS1 --set "scaling mode" "Center"
However, it is not possible to do this for external panels (VGA, DVI, HDMI, DP). The "scaling mode" property is not exported for these panels. There are many use cases for it, mainly related to gaming. Also some medical software like electrocardiogram monitor can't be stretched.
The function is implemented on radeon drivers already.
(I simply copied the above description from bug: 80868)