Xorg.conf required for configuring large desktops

[Kradorex Xeron]

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.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[John Russell (jrussell-88)]

I'm running Ubuntu 12.04 x64, kernel 3.2.0-24-generic with an NVidia GTX460 and an Achieva Shimian QH270-Lite 2560 x 1440 monitor connected by a dual-link DVI cable.

I've installed the nvidia-current 313-09 drivers from xorg-edgers (and tried the 310-19 from NVidia). Neither of these drivers read the EDID information from my monitor (/var/log/Xorg.0.log) and default to lower resolutions which my monitor will not display, resulting in a black screen.

If I replace the high-res monitor with a lower resolution 1600 x 900 monitor it works fine.

I want to force my video card to the resolution I need. NVidia's Xserver settings doesn't change anything. The xorg.conf configuration file is deprecated and the file is untouched by 12.04 - that is to say it doesn't access it - so I tried xrandr.

First I queried xrandr for a modeline for my resolution:
john@Vivid01:~$ cvt 2560 1440 60
# 2560x1440 59.96 Hz (CVT 3.69M9) hsync: 89.52 kHz; pclk: 312.25 MHz
Modeline "2560x1440_60.00" 312.25 2560 2752 3024 3488 1440 1443 1448 1493 -hsync +vsync

Then set up a new mode for it:
john@Vivid01:~$ xrandr --newmode "2560x1600_60.00" 348.50 2560 2760 3032 3504 1600 1603 1609 1658 -hsync +vsync

And attempted to attach it to a display:
john@Vivid01:~$ xrandr --addmode DVI-I-1 "2560x1440_60.00"
X Error of failed request: BadMatch (invalid parameter attributes)
  Major opcode of failed request: 153 (RANDR)
  Minor opcode of failed request: 18 (RRAddOutputMode)
  Serial number of failed request: 31
  Current serial number in output stream: 32

It appears xrandr won't accept the custom resolution.