Hello,
I'm using debian jessie with some backports too and I think I am
affected by the same underlying issue, I'm not quite sure if "change
linearly without light off" means what I'll describe as "flickering".
I am using an IBM Lenovo ThinkPad T61 and gnome 3.14.
When I upgraded xserver-xorg-video-intel from
2:2.99.917+git20160706-1~bpo8+1 to 2:2.99.917+git20161105-1~bpo8+1 I
noticed a "flickerig" in the backlight brightness whenever I changed
it via the hot keys. It is still occuring with
2:2.99.917+git20161206-1~bpo8+1.
As the upstream bug report has the staus NEEDINFO and it is requested
to "reproduce using *only*
/sys/class/backlight/intel_backlight/brigthness directly" I played
arround with different methods of brightness adjustion.
First of all a better description of what I mean with "flickering": on
each key press of the hot key the brightness is adjusted, however
there is a short period (maybe 1/10th of a second) where the backlight
is set to some other level. This is happening in both directions, but
not when the minimum or maximum is reached.
When I write values to either
/sys/class/backlight/intel_backlight/brigthness or
/sys/class/backlight/acpi_video0/brightness this flickering doesn't
occur. Also, when I fade the brightness in the gnome menu (upper right
corner for me, below the volume settings) it changes perfectly without
flickering. This lets me assume that is not a (pure) upstream issue,
but more likely an issue with the hot key management and its
interaction with xserver-xorg-video-intel (I assume that happens
somewhere else and no in xserver-xorg-video-intel, doesn't it?).
Therefore I share this findings here and not in the upstream bug. If
you'd like I can also post it there.
----
1. tune down brightness in gnome and hit brightnessdown key multiple
times to get to a dark state
root@my-host:~# cat /sys/class/backlight/acpi_video0/brightness
0
root@my-host:~# cat /sys/class/backlight/intel_backlight/brightness
0
2. tune brightness up to maximum in the gnome menu (backlight changes
seamlessly to the brightest state)
root@my-host:~# cat /sys/class/backlight/acpi_video0/brightness
0
root@my-host:~# cat /sys/class/backlight/intel_backlight/brightness
12056655
3. hit brightnessdown key few times (extreme flickering: backlight is
always shortly dark)
root@my-host:~# cat /sys/class/backlight/acpi_video0/brightness
0
root@my-host:~# cat /sys/class/backlight/intel_backlight/brightness
9042444
4. hit brightnessup key few times (less extreme flickering)
root@my-host:~# cat /sys/class/backlight/acpi_video0/brightness
2
root@my-host:~# cat /sys/class/backlight/intel_backlight/brightness
10248157
5. go back to default level, as in 1.
root@my-host:~# cat /sys/class/backlight/acpi_video0/brightness
0
root@my-host:~# cat /sys/class/backlight/intel_backlight/brightness
0
6. tune brightness up via echo to intel_backlight (backlight is in its
brightest state)
root@my-host:~# echo 12056655 > /sys/class/backlight/intel_backlight/brightness
root@my-host:~# cat /sys/class/backlight/acpi_video0/brightness
0
root@my-host:~# cat /sys/class/backlight/intel_backlight/brightness
12056655
7. hit brightnessdown key once (backlight is immediately dark)
root@my-host:~# cat /sys/class/backlight/acpi_video0/brightness
0
root@my-host:~# cat /sys/class/backlight/intel_backlight/brightness
0
8. tune brightness up via echo to acpi_video0 (backlight is in its
brightest state)
root@my-host:~# cat /sys/class/backlight/acpi_video0/brightness
15
root@my-host:~# cat /sys/class/backlight/intel_backlight/brightness
12056655
9. hit brightnessdown key once (backlight is immediately dark)
root@my-host:~# cat /sys/class/backlight/acpi_video0/brightness
14
root@my-host:~# cat /sys/class/backlight/intel_backlight/brightness
0
----
So the problem seems that the brightness levels are inconsistently
changed by the hot keys.
I noticed https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=801348#12
and added Option "Backlight" "intel_backlight" to the Device Section
in xorg.conf, however that made no difference.