Public bug reported:
Under Xubuntu, the built-in tools cannot be used to set many large
multi-monitor setups out of the box (in this example, on the restricted
driver on a Radeon HD 6750). Apparently a complete Xorg.conf file is
required with the "Virtual" keyword, though Xorg.conf seems to be
deprecated by recent versions of Xubuntu as this file doesn't exist by
default and everything is done within the window manager.
I was stepping someone through a procedure to get multiple monitors set
up under Xubuntu, however every time they would set a resolution above
an unspecified "Virtual" parameter, the GUI tools would provide no sign
of this error, just reverting back to a "clone" configuration. The only
clue that anything was happening was when xrandr was attempted, which is
counter-intuitive:
$ xrandr
Screen 0: minimum 320 x 200, current 1600 x 900, maximum 1680 x 1680
DFP1 disconnected (normal left inverted right x axis y axis)
DFP2 disconnected (normal left inverted right x axis y axis)
DFP3 connected 1600x900+0+0 (normal left inverted right x axis y axis) 443mm x
249mm
1600x900 60.0*+
1400x1050 60.0
1360x1024 60.0
1280x1024 60.0
1440x900 59.9
1280x960 60.0
1152x864 75.0 60.0
1280x768 60.0
1280x720 60.0
1024x768 75.0 70.1 66.0 60.0
800x600 72.2 75.0 60.3 56.2
640x480 75.0 72.8 67.0 59.9
DFP4 disconnected (normal left inverted right x axis y axis)
CRT1 connected 1600x900+0+0 (normal left inverted right x axis y axis) 474mm x
296mm
1680x1050 59.9 +
1400x1050 59.9
1600x900 59.9*
1360x1024 59.9
1280x1024 75.0 60.0
1440x900 59.9
1280x960 60.0
1280x800 60.0
1152x864 59.9 75.0
1280x768 60.0
1280x720 60.0
1024x768 75.0 70.1 60.0
800x600 72.2 75.0 60.3 56.2
640x480 75.0 72.8 67.0 59.9
CRT2 disconnected (normal left inverted right x axis y axis)
$ xrandr --output DFP3 --right-of CRT1
xrandr: screen cannot be larger than 1680x1680 (desired size 3200x900)
This is a bug since xorg.conf does not in fact exist on a default
Xubuntu configuration and seems to be deprecated by dynamic
configuration tools that work within the WM and additionally, graphical
configuration tools do not provide information on the limitations, below
is an example of how Xorg.conf does not exist:
$ stat /etc/X11/xorg.conf
stat: cannot stat `/etc/X11/xorg.conf': No such file or directory
A complete xorg.conf should not have to be written just to over-ride an
arbitrarily calculated Virtual desktop space, this should be dynamic as
the resolution requires. The "Virtual" keyword requirement seems to be
remnants from the time where an xorg.conf file was required and X
couldn't operate without it.
** Affects: xrandr (Ubuntu)
Importance: Undecided
Status: New
** Tags: resolution xrandr xserver-xorg xubuntu
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which is subscribed to xrandr in Ubuntu.
https://bugs.launchpad.net/bugs/938406
Title:
Xorg.conf required for configuring large desktops
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