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ATI M22 mobility radeon x300 and external monitors

26 Oct 2009


The Radeon X300 seems to have ’two crtc’ interfaces only. And one is always used when the LVDS (laptop builtin display) is enabled (in use). So one is left. Since i use DVI-0 on one workplace and VGA-0 on the other i usually was forced to reset X to make things work, since i wasn’t aware of the resource limitation. The error which came up was:

% xrandr --auto --output VGA-0 --mode 1024x768
xrandr: cannot find crtc for output VGA-0

The solution is easy. Just disable (in this case) DVI-0 output. Next enable VGA-0 output again.

% xrandr --output DVI-0 --off
% xrandr --auto --output VGA-0 --mode 1280x1024 --right-of LVDS

Usability wise this problem is a result of a missing resource counter (maybe it exists) but still the interface ‘xrandr’ was not very helpful in diagnosing the issue. I would have expected an error message as:

xrandr: no free crct for output VGA-0 left. you can only enable two devices at a time, maybe disable one to enable the other.

lspci shows:

# lspci
00:00.0 Host bridge: Intel Corporation Mobile **915GM**/PM/GMS/910GML Express Processor to DRAM Controller (rev 03)
01:00.0 VGA compatible controller: ATI Technologies Inc **M22** [Mobility Radeon **X300**]
00:01.0 PCI bridge: Intel Corporation Mobile **915GM/PM** Express PCI Express Root Port (rev 03)

Thanks to [1]