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Bug 731504 - laptop panel backlight is dropped to 0 when external VGA display is connected
laptop panel backlight is dropped to 0 when external VGA display is connected
Status: RESOLVED NOTGNOME
Product: gnome-shell
Classification: Core
Component: general
3.12.x
Other Linux
: Normal normal
: ---
Assigned To: gnome-shell-maint
gnome-shell-maint
Depends on:
Blocks:
 
 
Reported: 2014-06-11 08:03 UTC by Anton Gubarkov
Modified: 2014-06-18 16:46 UTC
See Also:
GNOME target: ---
GNOME version: ---

Attachments
log of external monitor connection with drm.debug=0x4 and module backlight +fpt (68.81 KB, text/plain)
2014-06-11 08:06 UTC, Anton Gubarkov 		Details

Description Anton Gubarkov 2014-06-11 08:03:02 UTC 
Actual behavior:
Every time I connect an external display, my internal panel brightness is brought to 0. 

Expected behavior:
The internal panel brightness doesn't change when there is a desktop displayed on it.

I tried to debug this with kernel first. Kernel guy advised that this brightness change is requested from gdm.
Comment 1 Anton Gubarkov 2014-06-11 08:06:04 UTC 
Created attachment 278251 [details]
log of external monitor connection with drm.debug=0x4 and module backlight +fpt

brightness adjustment at Jun 11 10:29:04 was manual - I had to turn it up to see something
Comment 2 Ray Strode [halfline] 2014-06-11 17:00:44 UTC 
might be a driver issue.  Does it happen after you log in as well?
Comment 3 Anton Gubarkov 2014-06-12 06:33:08 UTC 
(In reply to comment #2)
> might be a driver issue.  Does it happen after you log in as well?

I'm not 100% sure I understood your question. What I observe happens when I'm logged in and I connect an external monitor to a VGA connector. 

When I connect VGA cable before powering on my laptop, I can observe a nice mirror of my internal screen on the external display - the text part of the boot sequence (both UEFI and after modsetting). As soon as gdm starts, the brightness of my internal panel is dropped to 0.

I turn it up by using the keys.

When I complete the logon and gnome-session starts, the brightness of my internal panel is dropped to 0 again.

I turn it up by using the keys.

When I go to display settings to re-arrange the mutual position of my monitors and apply the settings, the brightness of my internal panel is dropped to 0 again.

I also suspected the driver issue myself and consulted with the kernel developer who helped me to fix the brightness issue related to AC power plug/unplug. He believes that these brightness changes come from a user land. Please see the bug https://bugzilla.kernel.org/show_bug.cgi?id=77091#c30  

Please advise how I can provide more tracing info to you.
Comment 4 Bastien Nocera 2014-06-12 09:01:57 UTC 
gnome-settings-daemon doesn't touch the brightness when plugging or unplugging displays. This is either a driver regression, or a bug in gnome-shell, which handles brightness and display power on newer versions of GNOME.
Comment 5 Rui Matos 2014-06-12 09:36:44 UTC 
gnome-shell/mutter doesn't do anything automatically about the backlight.

Looking at the log, it seems that when you connect the VGA cable, the kernel does a full hotplug of *both* the eDP and VGA connectors. If that really is the case then someone should make sure that the previous backlight value for that connector is re-applied.

We might end up having to do this in mutter (perhaps by extending the monitors.xml file to include a backlight level?). But the kernel could probably be smarter here too if my assumption above is correct.
Comment 6 Anton Gubarkov 2014-06-16 06:45:59 UTC 
raised a kernel video driver bug. https://bugzilla.kernel.org/show_bug.cgi?id=78011