https://bugzilla.kernel.org/show_bug.cgi?id=57151
Summary: Backlight controls do not work
Product: ACPI
Version: 2.5
Kernel Version: >=3.8.5
Platform: All
OS/Version: Linux
Tree: Mainline
Status: NEW
Severity: normal
Priority: P1
Component: Power-Video
AssignedTo: [email protected]
ReportedBy: [email protected]
Regression: No
I'm on a Dell XPS 13 running Archlinux with Intel chipset ad Intel Graphics HD
4000 video card.
With kernel versions <= 3.8.4, all backlight controls work properly: the slider
in the battery plasma widget, hot keys on the keyboard and also echoing
directly into /sys/class/backlight/acpi_video0/...
If I update to a version >= 3.8.5 [tested up to 3.8.8] backlight goes crazy and
sets brightness to 100% at startup; moreover, none of the mentioned controls
will work and thus it will be impossible to reduce brightness.
The other day, a strange thing happened: I was in the described situation and
was trying to manually set the brightness with echo.
Now, in my /sys/class/backlight/ there are two directories, acpi_video0 and
intel_backlight.
I tried to echo a value into intel_backlight/brightness and the screen started
flashing, as if it was trying to repeatedly update both the value in
acpi_video0 and the one in intel_backlight.
After a couple more echoes, backlight controls [all of them] simply started
working properly, until the following reboot. I didn't try to reproduce this
last episode.
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