Detecting if display is powered on or not

Alex Deucher alexdeucher at gmail.com
Wed Oct 20 12:46:33 PDT 2010


On Wed, Oct 20, 2010 at 1:38 PM, Pasi Kärkkäinen <pasik at iki.fi> wrote:
> 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? :)

Nope.

Alex

>
> -- 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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>> > dri-devel at lists.freedesktop.org
>> > http://lists.freedesktop.org/mailman/listinfo/dri-devel
>> >
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>


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