xrandr weirdness: cannot find crtc

[Alex Deucher]

On Wed, Jun 17, 2009 at 3:28 AM, Gilad Arnold<arnold at cs.berkeley.edu> wrote:
> Dear all,
>
> This looks like a driver related issue, apologies if it isn't.
>
> I'm running xf86-video-ati 6.12.2 on a Thinkpad T40. After startup the
> X server is configuration looks as follows:
>
> $ xrandr
> Screen 0: minimum 320 x 200, current 1400 x 1050, maximum 1920 x 1200
> VGA-0 connected 1400x1050+0+0 (normal left inverted right x axis y axis) 546mm x 352mm
>   1920x1200      60.0 +   60.0
>   1600x1200      75.0     70.0     65.0     60.0     60.0
>   1680x1050      60.0
>   1600x1000      60.0
>   1400x1050      74.8     60.0*
>   1280x1024      85.0     75.0     60.0
>   1280x960       85.0     60.0     60.0
>   1152x864       75.0
>   1024x768       85.0     75.1     75.0     70.1     60.0
>   832x624        74.6
>   800x600        85.1     72.2     75.0     60.3     56.2
>   640x480        85.0     75.0     72.8     75.0     60.0     59.9
>   720x400        85.0     70.1
>   640x400        85.1
>   640x350        85.1
> DVI-0 connected 1400x1050+0+0 (normal left inverted right x axis y axis) 546mm x 352mm
>   1920x1200      60.0 +
>   1600x1200      60.0     60.0
>   1680x1050      60.0
>   1600x1000      60.0
>   1400x1050      74.8     60.0*
>   1280x1024      85.0     75.0     60.0
>   1280x960       85.0     60.0     60.0
>   1152x864       75.0
>   1024x768       85.0     75.1     75.0     70.1     60.0
>   832x624        74.6
>   800x600        85.1     72.2     75.0     60.3     56.2
>   640x480        85.0     75.0     72.8     75.0     60.0     59.9
>   720x400        85.0     70.1
>   640x400        85.1
>   640x350        85.1
> LVDS connected 1400x1050+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
>   1400x1050      50.0*+
>   1280x1024      59.9     60.0
>   1280x960       60.0     59.9
>   1280x854       59.9
>   1280x800       59.8
>   1280x720       59.9
>   1152x768       59.8
>   1024x768       60.0     59.9
>   800x600        60.3     59.9     56.2
>   640x480        59.9     59.4
> S-video disconnected (normal left inverted right x axis y axis)
>
>
> Running xrandr with --verbose reveals that VGA-0 and DVI-0 share crtc
> 1 (clones) while LVDS uses crtc 0. Now I try to change mode on DVI-0 but
> the command fails with an error message:
>
> $ xrandr --output DVI-0 --mode 1680x1050
> xrandr: cannot find crtc for output DVI-0
>
> Same happens with different modes (including --auto) and with VGA-0. To
> work around the problem I have to turn off one of the two, then change
> mode on the other (it has to be two separate commands, otherwise the
> same error occurs):
>
> $ xrandr --output VGA-0 --off
> $ xrandr --output DVI-0 --mode 1680x1050
>
> Any idea what's causing this? I recall this used to work with earlier
> versions of the driver.

When the one crtc drives several monitors the mode timing must be the
identical on both. I suspect the mode timings aren't quite the same
across monitors for the modes you are trying to set.  Alternatively,
xrandr isn't being smart about how it sets the mode which results in
the error.

Alex