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I've been trying to set up an external monitor for a bit, finally got it mostly working except the resolution is off. My monitor's native res is 1600x900, which I finally got to show up in xrandr:

user:~$ xrandr
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 640 x 480, current 1024 x 768, maximum 1600 x 900
default connected 1024x768+0+0 0mm x 0mm
   1024x768        0.0* 
   800x600         0.0  
   640x480         0.0  
   1600x900       59.9

However when I try to change the resolution I get the following error:

user:~$ xrandr --output default --mode 1600x900
xrandr: Failed to get size of gamma for output default
xrandr: Configure crtc 0 failed
share|improve this question
The crtc is a part of your graphics adapter that decides what pixels should be what color as your screen is painted on your monitor (it does all of the timing)--so this may mean that though you monitor may be able to use this mode, your graphics adapter can't. It might be helpful to know about your equipment, the contents of /var/log/Xorg.0.log, and know what you had to do to get that resolution into xrandr's display. The other 0.0 displays look strange, by the way. – John S Gruber May 19 '12 at 3:15

I found that problem with ATI video drivers, with deactivated driver all works.


xrandr --verbose

to determine your ctrtc number? And try correct output syntax, e.g.:

xrandr --crtc CRT1 --output CRT1 --brightness 0.7

other outputs:

  • the Intel driver uses LVDS, VGA, TMDS-1 (TMDS-2, ...), TV
  • the ATI driver uses LVDS, VGA-0 (VGA-1, ...), DVI-0 (DVI-1, ...), S-video
  • the RadeonHD driver uses PANEL, VGA_1 (VGA_2, ...), DVI-I_1/digital or DVI-I_1/analog (DVI-I_2/digital or DVI-I_1/analog, ...), TV_7PIN_DIN
  • the NV driver uses LVDS, VGA0 (VGA1, ...), DVI0 (DVI1, ...), ???
  • the MGA driver uses ???, VGA (or VGA1, VGA2, ...), DVI (or DVI1, DVI2, ...), ???
  • the Nouveau driver uses LVDS, VGA-1, DVI-I-1, HDMI-1, ???
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"error: crtc must be used after output" And even specifying the CRTC shows by xrandr, I still get the error "Configure crtc 0 failed" – Cerin Apr 22 '15 at 4:54
Agreed. You have to specify the output first: "xrandr --output CRT1 --crtc CRTCNUMBER --brightness 0.7". As to why this didn't work for you, however, no clue. – BobDoolittle Dec 15 '15 at 6:12

xrandr --verbose was very helpful to me. i managed to create a few scripts to help out with my buggy docking station situation. i want docked to be only external display, undocked to mean only laptop.

xrandr -d :0 --output eDP1 --mode 1920x1080 --crtc 1 \
    --output DP1-1 --off \
    --output DP1-2 --off \
    --output DP1-3 --off

and also...

xrandr -d :0 --output DP1-1 --mode 1920x1080 --crtc 1 \
    --output eDP1 --off \
    --output DP1-2 --off \
    --output DP1-3 --off
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