Comment 48 for bug 573984

Here's how I experience this bug on an Asus UL30A.
The bug is in...

* 2.6.34 (vanilla)
* 2.6.35.4 (vanilla)
* 2.6.35-21-generic (after upgrading to ubuntu 10.10beta)
* 2.6.35-22-generic

It occurs with...

* default kernel parameters
* acpi_backlight=video
* acpi_osi=Linux

== bug experience #1 ==

And in all these cases I see the following output:
$ cat /proc/acpi/video/VGA/LCDD/brightness
levels: 5 11 17 23 29 35 41 47 53 59 65 72 79 86 93 100
current: 59

The value seems to be chosen randomly from this list after booting. I can set it manually only from 5 to 47. If I want to set a higher value, I get a "invalid value" error. And best of all: the values are not linear! Screen brightness is higher at 5 than at 11, but goes higher at 17 than at 11, and so on.
When trying to change the brightness with the Fn keys of my laptop, there's a KDE app showing brightness. But it doesn't change... it somehow seems to interrupt the system, because everything's a bit slower for a second and the brightness app doesn't disappear as soon as it normally does. But after a few seconds, everything's fine - except that the brightness has a new random value. It might be brighter or darker, no matter which key I pressed.

== bug experience #2 ==

And then there's a slightly different experience of this bug, when I use

* acpi_backlight=vendor

Here I can't use the /proc file, because:
$ cat /proc/acpi/video/VGA/LCDD/brightness
<not supported>

But the KDE brightness app works (or does not) the same way as before.

== bug experience #3 ==

And finally another slightly different experience of this bug. When I use the following kernel parameter...

* acpi_osi=

...I can change the brightness successfully and in a linear way when using the Fn keys. They work fine. Unfortunately the KDE brightness app seems to be gone and I also can't use /proc, because:

$ cat /proc/acpi/video/VGA/LCDD/brightness
<not supported>

== laptop details ==

Asus UL30a

00:02.0 VGA compatible controller: Intel Corporation Mobile 4 Series Chipset Integrated Graphics Controller (rev 07)
00:02.1 Display controller: Intel Corporation Mobile 4 Series Chipset Integrated Graphics Controller (rev 07)

dmesg doesn't show any useful information related to this stuff.