load_detection in xorg.conf?

Alex Deucher alexdeucher at gmail.com
Mon Nov 26 08:32:03 PST 2007

On Nov 26, 2007 11:24 AM, Lothar Brendel <l.brendel at goldmail.de> wrote:
> Alex Deucher wrote:
> > On Nov 26, 2007 4:30 AM, Lothar Brendel <l.brendel at goldmail.de> wrote:
> >> Hi folks!
> >>
> >> Do I understand it correctly that enforcing "load_detection=1" for
> >> an output (in my case a VGA-TFT is not detected at DVI-0 of an
> >> Radeon 9600, probably because of the adapter) is one of the things
> >> that *cannot* be done in xorg.conf? Thus, I have to put the
> >> corresponding xrand command into the [gkx]dm-scripts?
> >
> > If you have a new enough version of the ati driver you can specify it
> > as a driver option.   6.7.196 should have it.
> > Option "TVDACLoadDetect" "true"
> Thanx a lot, I'll try it as soon as 6.7.196 makes it into the Ubuntu
> repository (it's not that terribly urgent as to make self compilation
> necessary).
> But I have to admit some confusion: The docs
> (http://cgit.freedesktop.org/xorg/driver/xf86-video-ati/tree/man/radeon.man)
> explain the option as:
> } Enable load detection on the TV DAC.  The TV DAC is used to drive both
> } TV-OUT and analog monitors. Load detection is often unreliable in the
> } TV DAC so it is disabled by default.
> In my case, the analog output VGA-0 is connected to an analog projector and
> is detected and driven w/o problems. Can't I then conclude from the above
> that the TV DAC is running already? So how does the option "know" to switch
> on load_detection on the output DVI-0?

Most non-IGP radeons have 2 dacs (primary and tv).  Load detection
works reliably on the primary dac, but not on the tv dac.  By default
load detection is enabled for the primary dac, but not for the tv dac.
 load detection is used to detect the presence of an analog monitor
when DDC is not available.  In most cases with DVI + VGA cards, the
primary dac drives the VGA port and the analog portion of the DVI port
is driven by the TV dac.


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