Problem setting up dualhead with notebook and xrandr

[Alex Deucher]

On Wed, Jan 7, 2009 at 5:35 AM,  <kzot at gmx.net> wrote:
> Hi,
>
> I switched over from radeohd to radeon/ati for testing and encountered two
> problems with dualhead setup and xrandr; maybe someone here can me help out a
> bit.
>
> Setup is as follows: T60 Notebook in docking station (display "LVDS" and
> resolution 1400x1050) and two external displays via DVI (display "DVI-0" and
> resolution 1280x1024) and VGA (display "VGA-0" and resolution 1280x1024).
>
> Goal is to disable the notebook display and have a dualhead on the two
> external displays.
>
> =1=
> xrandr initially reports back as maximum resolution 1400x1400 (the biggest
> screen). For dualhead on the two external display (2560x1024) to small.
>
> I have workarounded it by adding a subsection in my xorg.conf:
>
> Subsection "Display"
>  Virtual 2560 2560
> EndSubsection
>
> Just wondering what is needed to autodetect this, because radeonhd set it
> correctly to this without needing the hint of a subsction?
>
> =2=
> Trying to enable the dualhead on the external displays fails.
>
> I disabled the LVDS display via "xrandr --output LVDS --off" and tried to
> enable the external displays in dualhead mode "xrandr --output VGA-0 --left-of
> DVI-0". Which does not change anything.
>
> After toying around a bit with "xrandr --output X --pos XxY" it seems that
> both VGA-0 and DVI-0 are modified together when changing a parameter of one,
> e.g. setting "xrandr --output DVI-0 --pos 100x0" moves both DVI-0 and VGA-0
> 100 pixel right.
>
> What can be wrong here?

Both outputs are being driven by the same crtc (controls display
timing and fb offset).  This allows you to drive all three monitors at
the same time (LVDS on one crtc, DVI/VGA on the other).  To run them
on separate crtcs, specify the crtc with randr.  run xrandr --verbose
to see the crtc assignments.
xrandr --output LVDS --off
xrandr --output VGA-0 --crtc 0 --mode 1280x1024
xrandr --output DVI-0 --crtc 1 --mode 1280x1024

Alex