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1b1ed1a
(Get the code!)
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://
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.