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/
(In reply to comment #0) > 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. This is a debugging tool. All bets are off if you fiddle with the registers directly. We do not support this.
Use of freedesktop.org services, including Bugzilla, is subject to our Code of Conduct. How we collect and use information is described in our Privacy Policy.