Bug 76217

Created attachment 95875 [details]
Output of lspci, uname, xrandr, and package versions

I have a LED panel with a intel graphic card using the i915 module.

In this model, to avoid flickering of the screen, the Arch wiki[1] recommends to change the PWM modulation frequency, following this calculator [2].

In my case, I had to change the register 0xC8254 from its original value 0xD9C0D9C to 0x5730573 in order to have a PWM frequency of 700 Hz.

After I do the change, through the command
intel_reg_write 0xC8254 0x5730573
(part of the Intel GPU tools [3])
xbacklight start to behave wrong.

For example:
$ xbacklight -get
20.005741
$ xbacklight -set 30
$ xbacklight -get
11.997704
$ xbacklight -set 100
$ xbacklight -get
40.040184

The bight is modifyied according to the -set parameter, but then it detects that 40.040184 is the 100% (you can see that 30% of this value is aproximately the 11.997704 that I get before, and so on).

In addition, -inc and -dec do not work properly.
For examle:
$ xbacklight -get
20.005741
$ xbacklight -inc 10
$ xbacklight -get
11.997704
(and the bight when down).

I tried also by echoing directly to /sys/class/backlight/intel_backlight/brightness, with similar results:
 # cat /sys/class/backlight/intel_backlight/max_brightness
3484
# echo 3484 > /sys/class/backlight/intel_backlight/brightness
# cat /sys/class/backlight/intel_backlight/brightness
1395
# echo 2000 > /sys/class/backlight/intel_backlight/brightness
 # cat /sys/class/backlight/intel_backlight/brightness
800

I attach the output of some commands to identify my hardware. I am on an up-to-date Arch Linux, and the computer is an HP 9470m.

References:
[1] https://wiki.archlinux.org/index.php/backlight#Backlight_PWM_modulation_frequency_.28Intel_i915_only.29
[2] http://devbraindom.blogspot.fr/2013/03/eliminate-led-screen-flicker-with-intel.html
[3] http://cgit.freedesktop.org/xorg/app/intel-gpu-tools/