01-16-2013 02:46 PM - edited 01-16-2013 03:00 PM
ACPI capable operating-systems support _OSI-Method, which allow the BIOS or UEFI to ask the running operating-system for its identity. The vendors use this to hide or offer functionalities and also work around bugs. In most cases the vendors support only some specific versions of Windows, only some rare laptops support "Linux" (some exceptions are the t400 or R61!). So Linux relies normally on a little white lie, it tells the BIOS or UEFI it is some recent version of "Windows".
To support officially Windows 2012 (better known as Windows 8) Lenovo changed the behaviour of its UEFI (Release 2.0 and everthing above for 2012 models?). Windows 2012 requires support for at least 101 different brigthness-levels, which are - obviously - not supported by recent ThinkPads. As far as we know from dumps of the ACPI-Table, the engineers decided to map the 16 existing brigthness-levels on the 101 required brigthness-levels. In meanwhile the developers of Linux started to report itself as "Windows 2012" with Kernel 3.7.0 for better supporting current hardware.
This has broken the backlight on some, if not all, current ThinkPad-Models. In default configuration, it is no longer possible to dim the backlight. It looks like this can be fixed with some quirks. To make the best of this situation, I ask the engineers/support for some information:
https://bugzilla.kernel.org/show_bug.cgi?id=51231 # the bug itself and discussion
http://lwn.net/Articles/291498/ # ACPI OSI explained (pdf is down, but latex is embedded)
http://msdn.microsoft.com/en-us/library/windows/hardware/jj128256.aspx # microsofts requirements for Windows 8, search for "101" and you will find the section
Thank you for your help
I think it would be the best thing, if you post your answers here or at the mentioned bug some lines above. I decided not to write a email to an "official" address.