We already know what the offending commit is.
commit a57f7f9175b8ccbc9df83ac13860488913115de4
Author: Bob Moore <[email protected]>
Date: Fri Aug 17 10:55:02 2012 +0800
ACPICA: Add Windows8/Server2012 string for _OSI method.
The problem is that when Linux starts reporting itself as Windows 8 some
Lenovo firmware changes its behavior to indicate that the ACPI backlight
interface reports 101 brightness levels (this is a requirement of
Windows 8). But in reality the firmware ignores 85% of the values that
it says are valid and only accepts the same 16 values that it says are
valid when the OS reports it self as something other than Windows 8. Any
other values are completely ignored.
I've gone back and forth with upstream on this issue, but the
maintainers haven't been very responsive. I've got a patch series that's
been sitting on the mailing list for a couple of weeks now without any
review from the maintainers.
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https://bugs.launchpad.net/bugs/1098216
Title:
Regression in brightness control on Lenovo Thinkpad X230 and X1 Carbon
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