[ltp] brightness control erratic on T430s with 3.10 kernel

[Tamas Papp]

Hi,

Since switching to the 3.10-2 from 3.9-1, I noticed that brightness keys
don't change the LCD brightness of my T430s gradually, only in large
steps, which is mapped rather erraticaly to what Linux thinks the
brightness is.

When I use the brightness keys, 

/sys/class/backlight/acpi_video0/brightness
and
/sys/class/backlight/acpi_video0/actual_brightness

do change, but the LCD backlight remains unchanged.  Then suddenly at
around 50 it changes by a large step.

--8<---------------cut here---------------start------------->8---
tamas@tamas:~$ uname -a
Linux tamas 3.10-2-amd64 #1 SMP Debian 3.10.7-1 (2013-08-17) x86_64 GNU/Linux
tamas@tamas:~$ cat /sys/class/backlight/acpi_video0/max_brightness 
100
--8<---------------cut here---------------end--------------->8---

and with the 3.9 kernel,

--8<---------------cut here---------------start------------->8---
tamas@tamas:~$ uname -a
Linux tamas 3.9-1-amd64 #1 SMP Debian 3.9.8-1 x86_64 GNU/Linux
--8<---------------cut here---------------end--------------->8---

Does anyone have a fix, or some hints on how to debug this (eg maybe I
could submit a bug report, but I need more specific info I guess).

I found some bug reports that could be related, eg
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1098216
but I am unsure if this is the same bug.

Best,

Tamas