Detecting if display is powered on or not

[Alex Deucher]

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.

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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