I'm running Ubuntu 16.04 on my Acer Aspire 7750G laptop with switchable graphics (integrated Intel graphics and an AMD Radeon HD 6650M). I recently bought an AOC Q3279VWF 2560x1440 monitor that I connected to the laptop through HDMI.
The largest possible resolution for the monitor under Display settings was 1920x1080. To force the native 2560x1440 I messed around with cvt and xrandr. Most refresh rates did not work (displayed an error in the terminal or no signal on the monitor) but when I by accident tried 44 Hz it suddenly worked:
cvt 2560 1440 44
xrandr --newmode "2560x1440_44.00" 222.75 2560 2720 2992 3424 1440 1443 1448 1479 -hsync +vsync
xrandr --addmode HDMI-1 "2560x1440_44.00"
However, while I get the correct resolution on the monitor, there is a constant terrible lag on everything that moves on the screen; when watching a video it is very stuttery; when moving the cursor it lags behind.
What is the reason for this lag and can it be fixed? Also, is there a logical pattern for which refresh rates work and which don't?
After reading this article on Medium I downloaded
umcwhich I used with the--rbtoption to generate the mode line instead ofcvt. I adjusted the frame rate to get close to the same pixel clock that was used when it was working (but stuttering) which was222.75.This lead me to use
that has a pixel clock of
221.00.After adding this mode it worked like a charm, with the correct resolution and without any lag or stutter.
The whole process and solution seemed kind of random though...