How does the screen backlight work?

Question

Everything about the backlight settings and controls work for me except:

Backlight resets to max. on every reboot/boot. The backlight minimum goes on to complete black screen, instead of the minimum brightness setting elsewhere. NB> I see that 1. above is answered on a lot of places with different answers, so I am actually looking for someone or somplace where I can read and understand how it all works.

I have 2 different backlight folders in my laptop and many conf files to edit. So need to understand what on those files impacts what on the sytem.

Answer 1

At the heart of backlighting is this Linux Kernel parameter that's exposed to you through here under /sys. You can manipulate it by setting the value to something between 1 and 15. For example:

$ echo 5 | sudo tee /sys/class/backlight/acpi_video0/brightness

Set's the brightness to 5. Manipulating this Kernel parameter is abstracted away so that when you're changing the value with your keyboard or a desktop applet you're manipulating it through D-Bus and HAL.

D-Bus is allowing you to manipulate this structure, org.freedesktop.Hal.Device.KeyboardBacklight, and HAL is allowing the privilege to do so. You can see this on my Fedora 14 system like this:

$ grep -i backlight /etc/dbus-1/system.d/*
/etc/dbus-1/system.d/hal.conf:         send_interface="org.freedesktop.Hal.Device.KeyboardBacklight"/>
/etc/dbus-1/system.d/hal.conf:         send_interface="org.freedesktop.Hal.Device.KeyboardBacklight"/>

In the file hal.conf:

  <!-- Only allow users at the local console to manipulate devices -->
  <policy at_console="true">
  ...
      <allow send_destination="org.freedesktop.Hal"
           send_interface="org.freedesktop.Hal.Device.KeyboardBacklight"/>

You can query the current value, through D-Bus like so:

$ dbus-send \
     --print-reply \
     --system \
     --dest=org.freedesktop.Hal  \
     /org/freedesktop/Hal/devices/computer_backlight \
     org.freedesktop.Hal.Device.LaptopPanel.GetBrightness | \
     tail -1 | \
     awk '{print $2}'

Which returns the value:

15

You can also manipulate it from the command line like so, (the bit int32:10 below is setting brightness to "10"):

$ dbus-send \
     --print-reply \
     --system \
     --dest=org.freedesktop.Hal  \
     /org/freedesktop/Hal/devices/computer_backlight \
     org.freedesktop.Hal.Device.LaptopPanel.SetBrightness \
     int32:10 #2&>1 > /dev/null

You can see that we changed the brightness:

$ cat /sys/class/backlight/acpi_video0/brightness
10

So how do I fix this?

One idea would be to save the current brightness out to a file prior to either a shutdown and/or reboot and then add to your startup (perhaps ~/.xinitrc) the dbus-send ... command above adding in the brightness value you previously saved out to the file.

Why do I have multiple files under /sys/class/backlight?

I came across this Q&A on askubuntu.com titled: Why there are two brightness control file (/sys/class/) in my system. In the answer to this there was this comment:

If the system starts with the kernel parameter acpi_backlight=vendor, the item acpi_video0 is replaced by the item intel, but then the Fn-Keys can not change the value of this item.

I also came across this documentation for the Kernel, titled: Kernel Parameters. In this doc the following aCPI options are mentioned:

acpi_backlight= [HW,ACPI]
        acpi_backlight=vendor
        acpi_backlight=video
        If set to vendor, prefer vendor specific driver
        (e.g. thinkpad_acpi, sony_acpi, etc.) instead
        of the ACPI video.ko driver.

I think the intel_backlight referenced in /sys/class/backlight is part of the backlighting for the video card drivers provided for Intel graphics cards.

References

• Save brightness setting on reboot in Debian Squeeze / Wheezy
• DBUS Backlight Brightness