Thursday, June 20, 2013

ACPI Brightness Control (4) - Workaround for Brightness Problems in Ubuntu Linux

This blog discusses some common errors for brightness control and hotkeys. It will be updated from time to time.

Initial Zero Brightness

Some system BIOS reports zero in _BQC at boot-up, and it causes Linux kernel to dim the screen to minimum. In some cases, the panel is completely black out. This is because Linux's ACPI VGA driver does the following when it starts:

  1. Read _BQC for current brightness level
  2. Set brightness level to maximum - it is to verify whether brightness work
  3. Restore brightness level from 1
Aside from getting BIOS fixes which is usually close to impossible as the product is shipped (but I do encourage people to call customer services to complain!), there are three ways to fix it:

Kernel parameter

Edit /etc/default/grub
Add "video.use_bios_initl_backlight=0" to GRUB_CMDLINE_LINUX_DEFAULT
Run "sudo update-grub"

Bootup script

Edit the file /etc/rc.local
Add a line "echo 4 > /sys/class/backlight/acpi_video0/brightness" before "exit 0"

Quirk to acpi/video.c

If you are familiar with Linux kernel, you can develop a patch as in If not, it is more than welcomed to file a bug like LP#1184501, run "sudo dmidecode > dmi.log", attach dmi.log and assign the bug to me. I am very happy to develop a patch for it, and you may help many others using Linux!

Brightness Fails to Work

If you are have laptop PC that BIOS fails to adjust brightness via acpi_video, you can try add the kernel parameter "acpi_backlight=vendor".

What the parameter does is to stop using acpi video to adjust the brightness - also skips creating sysfs nodes for acpi_videoX. As a result, a desktop daemon (like GSD) will find other brightness sysfs nodes created by other drivers, such as Intel_backlight, thinkpad_backlight and so on.

A Hotkey Press Changes Two Levels

In Figure 2 of ACPI Brightness Control (3) - Hotkeys in Ubuntu Linux, a solid and a dash line were draw to call to BIOS _BCM. This implies a hotkey can and may change a brightness twice. This is the case with Gnome-Setting-Daemon in Ubuntu.

If seeing this problem, one can add an kernel parameter to workaround it:
  • sudo vi /etc/default/grub
  • add brightness_switch_enabled=N to GRUB_CMDLINE_LINUX_DEFAULT
  • sudo update-grub
  • reboot

This will skip the dashed line call in ACPI Video driver:



1 comment:

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