On Sunday, June 09, 2013 07:01:36 PM Matthew Garrett wrote:
Windows 8 introduced new policy for backlight control by pushing it out to graphics drivers. This appears to have coincided with a range of vendors adding Windows 8 checks to their backlight control code which trigger either awkward behaviour (Lenovo) or complete brokenness (some Dells). The simplest thing to do would be to just disable ACPI backlight control entirely if the firmware indicates Windows 8 support, but it's entirely possible that individual graphics drivers might still make use of the ACPI functionality in preference to native control.
The first two patches in this series are picked from other patchesets aimed at solving similar problems. The last simply unregisters ACPI backlight control on Windows 8 systems when using an Intel GPU. Similar code could be added to other drivers, but I'm reluctant to do so without further investigation as to the behaviour of the vendor drivers under Windows.
Well, after some more time spent on that, we now have a series of 3 patches (different from the $subject one) that we think may be used to address this issue. As far as I can say, it has been tested by multiple people whose systems have those problems and they generally saw improvement.
It is not my ideal approach, but it seems to be the least intrusive and/or with the least amount of possible side effects that we can do right now as a general measure (alternatively, we could create a possibly long blacklist table of affected systems with different workarounds for them, but let's just say that is not overwhelmingly attractive).
[1/3] Make ACPICA export things that we need for checking OSI(Win8).
[2/3] Make acpi_video_device_find_cap() call acpi_video_init_brightness() even if it is not going to register the backlight interface (needed for Thinkpads).
[3/3] Avoid using ACPI backlight if i915 is in use and the firmware believes we are Windows 8.
Many thanks to everyone involved!
Rafael