xrandr: how to tell it that a screen size is supported?

[J]

On Mon, Jun 7, 2010 at 09:04, Dotan Cohen <dotancohen at gmail.com> wrote:
> A Kubuntu 9.10 machine runs at 1680x1050 resolution with no problems
> if the KVM is switched to that machine when it is booted. If the KVM
> is switched to another machine, then this machine loads at a very low
> resolution and I cannot configure the proper resolution with xrandr:

> How can I either:
> 1) Configure the machine to always use 1680x1050 no matter how it is booted?
> -or-
> 2) Tell xrandr that 1680x1050 is fine?

A: this is because of your KVM.  When you boot the machine without the
KVM pointing to the machine, X has no monitor to probe, so it defaults
to a lower safe setting.  When your KVM IS pointed to the system in
question, X get's supported modes from the monitor via a DDC probe and
thus, you get full resolution.

The obvious answer to this one is to only boot when the monitor is
connected to the system (e.g. when you have that system switched on
the KVM).

However, this is from the man page for xrandr (you should read the man
page, there's a bit of explanation for each xrandr option in there):

  Forces to use a 1024x768 mode on an output called VGA:
              xrandr --newmode "1024x768" 63.50  1024 1072 1176 1328
768 771 775 798 -hsync +vsync
              xrandr --addmode VGA 1024x768
              xrandr --output VGA --mode 1024x768

Of course, you'll have to figure out all the mode info for yourself,
because that's all dependent on your system and monitor...

So, the second answer is: 'man xrandr'

;-)

Cheers,
Jeff