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[Seth Forshee <seth.forshee+lp@xxxxxxxxxxxxx>]

The firmware has lots of strange complexities, but as far as I can tell
the acpi_video0 backlight interface should fundamentally work no better,
or possibly worse, when the patch is reverted. It would be instructive
if you could verify this by trying to adjust the brightness using some
mechanism other than the hotkeys, e.g. a brightness slider in the UI or
something like that.

In that case your backlight would probably be getting changed directly
by the firmware rather than by the operating system, and it could be
that this commit is responsible for the differing behaviors:

http://kernel.ubuntu.com/git?p=ubuntu/ubuntu-
saucy.git;a=commitdiff;h=efaa14c7e981bdf8d3c8d39d3ed12bdc60faabb8

If reverting this commit fixes your backlight when the other commit is
present then we'll know that's what is happening.

What this likely boils down to is that the ACPI backlight is broken in
weird ways, and Lenovo and/or their firmware supplier never bothered to
test it because they aren't using it in Windows. There's nothing in the
DMI data that I see which will allow us to distinguish between the
T430's for which the quirk works and those for which it does not. If you
were using the nouveau driver for your graphics you might have an
alternate backlight interface which worked, but the nvidia driver isn't
providing one.

What you can do as a workaround is to modify your setup to pass the
option acpi_osi="Windows 2012" to the kernel when booting, which ought
to override the quirk. I can't tell you how this would be done for your
distribution however.

-- 
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https://bugs.launchpad.net/bugs/1243267

Title:
  Backlight brightness control doesn't work

Status in “linux” package in Ubuntu:
  Confirmed

Bug description:
  Hello, during a kernel update from 3.11.0-4 up to 3.11.0-9 i've found out that my brightness controll of my Notebook (Lenovo Thinkpad T430-D17 (nvidia-graphic card, optimus disabled, driver version: 325.15) does not work). Therefore, i have tried several kernel from kernel.org where all Kernel provides the backlight control. To get comparable values, i have tried the kernel version 
  ubuntu-kernel: Ubuntu 3.12.0-0.2-generic and 3.12.0-rc6
  By using the the ubuntu kernel, i wasn't able to control the backlight brightness but by using the kernel profided by kernel.org works. Therefore, i began to search the patch lists of ubuntu and find this patch: http://kernel.ubuntu.com/git?p=ubuntu/ubuntu-trusty.git;a=blobdiff;f=drivers/acpi/blacklist.c;h=f49ffaedaefdaa28d533d339a3f5cda073687ed7;hp=9515f18898b2b578053c58309185daa7934cfdda;hb=02cb06962306d96f59bc97ec8289c14b73bafb4e;hpb=2eb9acd1a1deb3f4d6428d022fc43541c9e70069.
  I have reverted this patch and the brightness control of my notebook works fine. 

  If anybody wants to execute the ubuntu-bug program, you should note,
  that i'm not using ubuntu, i'm only using the ubuntu kernel and the
  ubuntu-bug program is not in my repositories. Therefore, i supply the
  lspci-vnvn report. If any other logs or reports are missing please
  feel free to ask me.

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