On the Lenovo Yoga 2 Pro, increasing the brightness to 100% using xbacklight, does not increase it to the maximum (duh!). How to reproduce: # Find out maximal brightness % cat /sys/class/backlight/intel_backlight/max_brightness 937 # Go to non 100% % xbacklight -set 50 % cat /sys/class/backlight/intel_backlight/brightness 381 # Go to 100% % xbacklight -set 100 % cat /sys/class/backlight/intel_backlight/brightness 765 # Increasing the brightness via sysfs actually increases the brightness even more % echo 937 > /sys/class/backlight/intel_backlight/brightness Software/Hardware: [11:26] artist:~% pacman -Q | grep -i -e xorg-server -e intel intel-dri 10.0.3-1 xf86-video-intel 2.99.910-1 xorg-server 1.15.0-5 xorg-server-common 1.15.0-5 xorg-server-devel 1.15.0-5 xorg-server-xnest 1.15.0-5 [11:26] artist:~% [11:26] artist:~% uname -a Linux artist 3.12.9-2-ARCH #1 SMP PREEMPT Fri Jan 31 10:22:54 CET 2014 x86_64 GNU/Linux [11:21] artist:~% lspci | grep -i vga 00:02.0 VGA compatible controller: Intel Corporation Haswell-ULT Integrated Graphics Controller (rev 09) [11:27] artist:~% lspci -n | grep 00:02.0 00:02.0 0300: 8086:0a16 (rev 09) [11:27] artist:~% CPU: Intel(R) Core(TM) i7-4500U
Check Xorg.0.log, it is probably using the acpi backlight interface instead. How that maps to the raw value is defined by your firmware (and in some extreme cases, the raw interface does not control the backlight at all).
Created attachment 94220 [details] xorg.0.log
It seems you are right: [ 18.869] (--) intel(0): found backlight control interface acpi_video0 (type 'firmware') Though I am wondering what it referes to by "firmware". So are we talking more about a kernel or xorg bug here?
Neither, just a misfeature in your ACPI firmware. The prevailing opinion is that the ACPI for all machines released in the last couple of years have a buggy backlight controller, and the kernel is striving to identify those broken interfaces and disable them.
A couple of temporary solutions: Option "Backlight" "intel_backlight" (in xorg.conf) video.use_native_backlight=1 (on kernel commandline - not certain about this one) At any rate, that the ACPI firmware fails to set the maximum brightness is an issue in Lenovo's implementation.
(In reply to comment #5) > At any rate, that the ACPI firmware fails to set the maximum brightness is > an issue in Lenovo's implementation. It's debatable whether ACPI firmware is wrong in limiting the brightness (both the maximum and the minimum non-off brightness), or our native backlight interface is wrong in *not* limiting the brightness according to the ACPI opregion. Some backlights might not like having 100% duty cycle, they might exhibit non-linear increase in power consumption, might have an impact on the longevity of the backlight, etc.
To move this forward you need to file a new bug report on bugzilla.kernel.org against the acpi video/backlight driver.
Created bug report at kernel.org; Reference: https://bugzilla.kernel.org/show_bug.cgi?id=72491
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