KDE can't change screen resolution in a VM

max.b · 11-08-2021, 11:05 AM

I'm trying a bunch of alternative desktops that come with Debian 11 in a VM (gnome-boxes).

All of the desktops I tried (XFCE, Gnome Flashback, Cinnamon) can change the screen resolution, but if I do it in KDE, the screen just flickers a bit and gives up -- doesn't even show a prompt to keep the new resolution.

Is this a known problem, and is there a solution to this?

frankbell · 11-08-2021, 07:27 PM

This is a shot in the dark, as I've never used gnome-boxes, but I have encountered a similar issue in VirtualBox a smattering of times.

I have been able to resolve it (with one exception, I think) by cycling through the display types (VMSVGA, VBoxVGA, VBoxSVGA, etc.) in the settings for the VM. I would shut down the VM, change the display type, and reboot to test the new selection.

mrmazda · 11-08-2021, 08:00 PM

Might be all that's needed is disabling KScreen.

max.b · 11-11-2021, 11:26 AM

Quote:

Originally Posted by frankbell

This is a shot in the dark, as I've never used gnome-boxes, but I have encountered a similar issue in VirtualBox a smattering of times.

I have been able to resolve it (with one exception, I think) by cycling through the display types (VMSVGA, VBoxVGA, VBoxSVGA, etc.) in the settings for the VM. I would shut down the VM, change the display type, and reboot to test the new selection.

I don't think Gnome Boxes has this option (I couldn't find it). Debian 11 dropped VirtualBox. So I can't use it (easily).

max.b · 11-11-2021, 11:27 AM

Quote:

Originally Posted by mrmazda

Might be all that's needed is disabling KScreen.

Isn't that what actually allows you to change screen resolutions in KDE?

I tried changing RAM from 2GB to 8GB, but that made no difference.

ferrari · 11-11-2021, 01:07 PM

I have openSUSE (with KDE desktop) running in a VM, and needed to provide an explicit display mode for it. I first used xrandr to create and set the required display mode (1920x1080) on the fly, and subsequently made it permanent with a minimal xorg.conf...

Code:

Section "Device"
  Identifier      "Configured Video Device" 
EndSection

Section "Monitor" 
  Identifier      "Configured Monitor"
# 1920x1080 59.96 Hz (CVT 2.07M9) hsync: 67.16 kHz; pclk: 173.00 MHz
Modeline "1920x1080_60.00"  173.00  1920 2048 2248 2576  1080 1083 1088 1120 -hsync +vsync
EndSection 

Section "Screen"  
  Identifier      "Default Screen" 
  Monitor         "Configured Monitor"
  Device          "Configured Video Device" 
  SubSection "Display"
    Modes "1920x1080_60.00" 
  EndSubSection
EndSection

Of course this approach will only work for desktops running within an X-server environment.

max.b · 11-11-2021, 01:30 PM

Quote:

Originally Posted by ferrari

I have openSUSE (with KDE desktop) running in a VM, and needed to provide an explicit display mode for it. I first used xrandr to create and set the required display mode (1920x1080) on the fly, and subsequently made it permanent with a minimal xorg.conf...

Code:

Section "Device"
  Identifier      "Configured Video Device" 
EndSection

Section "Monitor" 
  Identifier      "Configured Monitor"
# 1920x1080 59.96 Hz (CVT 2.07M9) hsync: 67.16 kHz; pclk: 173.00 MHz
Modeline "1920x1080_60.00"  173.00  1920 2048 2248 2576  1080 1083 1088 1120 -hsync +vsync
EndSection 

Section "Screen"  
  Identifier      "Default Screen" 
  Monitor         "Configured Monitor"
  Device          "Configured Video Device" 
  SubSection "Display"
    Modes "1920x1080_60.00" 
  EndSubSection
EndSection

Of course this approach will only work for desktops running within an X-server environment.

I remember doing stuff like this when installing Debian >10 years ago (and what drove me to Ubuntu at the time). But I thought we were past this?

mrmazda · 11-11-2021, 02:49 PM

Quote:

Originally Posted by max.b

Isn't that what actually allows you to change screen resolutions in KDE?

KScreen is KDE's tool for managing X display configuration. It doesn't always work as expected. There are other tools, including the originals, /etc/X11/xorg.conf, and its newer variant, /etc/X11/xorg.conf.d/*.conf. It can take as little as the following /etc/X11/xorg.conf to override broken automagic failing to use a display's native 1920x1080 mode:

Code:

Section "Device"
	Identifier "DDev"
EndSection

Section "Monitor"
	Identifier	"DMon"
#	HorizSync	30-86	# may be needed if EDID broken
#	VertRefresh	56-76	# may be needed if EDID broken
	Option	"PreferredMode" "1920x1080"
EndSection

Section "Screen"
	Identifier	"DScr"
	Device		"DDev"
	Monitor		"DMon"
EndSection

ferrari · 11-11-2021, 04:18 PM

Quote:

Originally Posted by max.b

I remember doing stuff like this when installing Debian >10 years ago (and what drove me to Ubuntu at the time). But I thought we were past this?

For the most part yes (courtesy of EDID), but I just documented what I found necessary when VirtualBox didn't advertise the native display resolution of my laptop.