Quickfix: Ubuntu Hardy dual monitor with Intel 945GM troubles – can’t escape Clone mode

Problem:
On a Thinkpad T60 with Intel 945GM, Ubuntu 8.04 Hardy, the external VGA monitor is always cloned. Going into System->Preference->Screen Resolution lets you alter the resolution of the LCD and the external monitor, but it stays cloned whether “Clone Screens” is checked or not. Putting monitor to the left or right of the LCD does nothing (stays cloned). However, putting monitor above or below LCD does work (two desktops, one on top of the other).

Cause:
xrandr is complaining behind the scenes:

xrandr: screen cannot be larger than 1600x1600 (desired size 2560x1024).

The GUI offers no feedback that the operation has failed so it is quite confusing to the user. You can confirm this by trying manually, xrandr --output LVDS --left-of VGA and see if it complains.

Workaround:
NOTE: This workaround means you will lose direct rendering, so no Compiz or Google Earth. No big deal for me, but a dealbreaker for some I’m sure.

Edit /etc/X11/xorg.conf, and add the following subsections into Section "Screen":

EDIT: Subsection should have been SubSection. Thanks fahoo. (jb -20080501 2203 -0500)

SubSection “Display”
Virtual 2560 1024
EndSubSection


Virtual is the resolution of your double-monitor desktop: if you have an LCD at 1280×800 and a CRT to the right of it at 1280×1024, you would put 2560×1024 as in the above example. (Think x1+x2, max(y1,y2)).

Notes:
Note that the maximum supported size of the virtual desktop for the Intel 945GM series of chipset with 3D acceleration enabled, is 2048×2048. The virtual screen can be larger but DRI will be disabled.

Reference:
ThinkWiki – Xorg RandR 1.2

lspci:

$ lspci |grep -e VGA -e Display
00:02.0 VGA compatible controller: Intel Corporation Mobile 945GM/GMS, 943/940GML Express Integrated Graphics Controller (rev 03)
00:02.1 Display controller: Intel Corporation Mobile 945GM/GMS/GME, 943/940GML Express Integrated Graphics Controller (rev 03)