blank external DVI on Thinkpad T40

Alex Deucher alexdeucher at
Wed Feb 25 15:08:53 PST 2009

On Wed, Feb 25, 2009 at 5:51 PM, Gilad Arnold <arnold at> wrote:
> On Wed, Feb 25, 2009 at 05:25:14PM -0500, Alex Deucher wrote:
>> DVI is usually much pickier about modes than VGA.  you might try the
>> 59.9 mode on the DVI:
>> xrandr --output DVI-0 --mode 1920x1200 --rate 59.9
> I get the same result (i.e. no picture on the DVI) except that my
> picture quality deteriorates even more on the VGA with 59.9  (for some
> reason, changing the mode of the DVI-0 changes the mode of the VGA-0
> accordingly...?)

There are only two display controllers (crtcs) so if you have more
than two outputs, some of them will be driven by the same display
controller.  In this case the one crtc is driving the LVDS and the
other crtc is driving both the DVI and the VGA port since they are
both connected.

>> from the hardware's perspective they do the same thing: turn off the
>> output and disable the crtc driving it.
> Does xorg/driver rearrange the video memory layout when xrandr disables
> a display with the --off switch, even though the monitor is still
> connected?  I'm close to moron when it comes to video controllers and
> being purely speculative, but I suspect that --off only shuts down the
> signal/CRTC but retains and updates the video RAM allocated to the LVDS,
> thus not helping to eliminate the problem (be it memory overflow,
> bandwidth limitation, etc).

Vram isn't allocated to specific crtcs, it's more like the crtcs point
at a location in vram; like a window into vram.  They can point to
different places (dualhead) or the same place (clone).  "--off"
disables the crtc (turns it off and removes it's requests from the
memory controller) and any outputs driven by that crtc.

>> I'll take a look and let you know.
> Million thanks. This has been very frustrating, and your help is very
> appreciated.

No problem.


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