Detecting if display is powered on or not

[Pasi Kärkkäinen]

On Sun, Oct 17, 2010 at 03:25:09PM -0400, Alex Deucher wrote:
> On Sun, Oct 17, 2010 at 9:07 AM, Pasi Kärkkäinen <pasik at iki.fi> wrote:
> > Hello,
> >
> > While I was discussing the laptop lid/dock issues with Fedora
> > people this issue was brought up.
> >
> > How do you determine if a display is actually powered on or not?
> >
> > Scenario:
> > - Laptop in a docking station, lid open, so internal LVDS enabled.
> > - External DVI display connected, but powered off.
> >
> > So in this case the external display shouldn't be used, since it's off.
> >
> > What happens is this:
> >
> > # cat /sys/class/drm/card0-DVI-D-1/status
> > connected
> >
> > # cat /sys/class/drm/card0-DVI-D-1/enabled
> > enabled
> >
> > # cat /sys/class/drm/card0-DVI-D-1/modes
> > 1920x1080
> > 1600x1200
> > 1680x1050
> > 1280x1024
> > 1440x900
> > 1280x960
> > 1280x800
> > 1024x768
> > 800x600
> > 800x600
> > 640x480
> >
> > So it *looks* like it's enabled and active, but actually it's powered off.
> > Is there a method to determine the actual state?
> >
> 
> Unfortunately not.  Monitors are designed to keep the EDID powered up
> even if the monitor is powered off so that graphics chips can detect
> that something is attached.  There's no way to know whether the
> monitor is on of off however.
> 

Ok. Understood.

And there are no other protocols that would reply only when the monitor is actually on? :)

-- Pasi

> Alex
> 
> > That is pretty important information for the daemon/tool/script implementing
> > the "policy" of which outputs to enable/disable..
> >
> > Thanks!
> >
> > -- Pasi
> >
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> >
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