Please attach the acpidump output.
you can use the latest pmtools at
http://www.lesswatts.org/patches/linux_acpi/

Created attachment 17371 [details]
ACPI dump

                    Method (_BCL, 0, NotSerialized)
                    {
                        Return (Package (0x10)
                        {
                            0x0F,
                            0x0E,
                            0x0D,
                            0x0C,
                            0x0B,
                            0x0A,
                            0x09,
                            0x08,
                            0x07,
                            0x06,
                            0x05,
                            0x04,
                            0x03,
                            0x02,
                            0x01,
                            0x00
                        })
                    }

weird, I find a really buggy _BCL method in your BIOS, which is used for brightness control.

Please attach the content of /proc/acpi/video/VGA/LCDD/brightness.

By reading ACPI spec ACPI Extensions for Display Adapters B.6.2,
we can get this, "Each brightness level is represented by a number between 0 and 100, and can be thought of as a percentage. For example, 50 can be 50% power consumption or 50% brightness, as defined by the OEM."
So this is clearly a violation of ACPI spec, which we won't fix in Linux/ACPI.
Please check if there are any new BIOS releases that have fixed the problem.

# cat /proc/acpi/video/VGA/LCDD/brightness 
levels:  15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
current: 13

Sorry for insisting on this, but at some point in the past this has worked correctly. Unfortunately I cannot pinpoint exactly what change in my system made this stop working... :/ Currently everything works, except the RandR part. Controling the brightness from the keyboard works, using /proc/acpi/video/VGA/LCDD/brightness also, but Gnome (which uses RandR by default) has its values all inverted. Are you sure this is a buggye BIOS?

Thanks in advance.

BTW, according to Packard Bell's site there is no new BIOS firmware version :/

First, I'd say that the ACPI video extension support in this BIOS is definitely buggy, no matter if it's the root cause of the inverted brightness issue.

(In reply to comment #5)
> # cat /proc/acpi/video/VGA/LCDD/brightness 
> levels:  15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
> current: 13
> 
> Sorry for insisting on this, but at some point in the past this has worked
> correctly.
> Unfortunately I cannot pinpoint exactly what change in my system
> made this stop working... :/
The issue should be consistent if using ACPI video driver and sys backlight class device.
Maybe you used some platform drivers to control the brightness?
what's your laptop model?

> Currently everything works, except the RandR part.
> Controling the brightness from the keyboard works, using
> /proc/acpi/video/VGA/LCDD/brightness also, but Gnome (which uses RandR by
> default) has its values all inverted. Are you sure this is a buggye BIOS?
Yes. RandR supports multiple modes and one of them is to use the sys backlight I/F to control the brightness. :)

please run "xrandr --output LVDS --set BACKLIGHT_CONTROL combination" and retry. :)

This is Packard Bell EasyNote BU45, lshal:

  system.chassis.manufacturer = 'Packard Bell BV'  (string)
  system.chassis.type = 'Notebook'  (string)
  system.firmware.release_date = '05/11/2007'  (string)
  system.firmware.vendor = 'American Megatrends Inc.'  (string)
  system.firmware.version = '206'  (string)
  system.formfactor = 'laptop'  (string)
  system.hardware.primary_video.product = 10146  (0x27a2)  (int)
  system.hardware.primary_video.vendor = 32902  (0x8086)  (int)
  system.hardware.product = 'EasyNote_BU45'  (string)
  system.hardware.serial = '000613360386'  (string)
  system.hardware.uuid = 'DC8A0100-3432-0180-BF5A-001D60001558'  (string)
  system.hardware.vendor = 'Packard Bell BV'  (string)
  system.hardware.version = 'PB72E06210'  (string)
  system.kernel.machine = 'x86_64'  (string)
  system.kernel.name = 'Linux'  (string)
  system.kernel.version = '2.6.26-1-amd64'  (string)

It's a quite cheap laptop, so I wouldn't be surprised that there are lots of bugs in it :/ I'm only using open source drivers, all stock Debian.

Besides "combination" I also set it to "legacy", "native" and back to "kernel". None worked.

Thanks for the help :)

Is there any path I can follow from here? Any configuration option which could overcome this?

Created attachment 17551 [details]
customized DSDT

this customized DSDT should work for you. :)
how to override a DSDT can be found at:
http://www.lesswatts.org/projects/acpi/overridingDSDT.php

That worked! Thank you very much for your help Zhang :)
Kind regards.