Detecting if display is powered on or not

[Pasi Kärkkäinen]

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? 

That is pretty important information for the daemon/tool/script implementing
the "policy" of which outputs to enable/disable..

Thanks!

-- Pasi