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SubjectRe: [Update][PATCH 0/3] Fix backlight issues on some Windows 8 systems
On Wed, Jul 17, 2013 at 7:16 PM, Rafael J. Wysocki <> wrote:
> 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!

I tried this patch series and it's as I expected, it's the same as
acpi_backlight=vendor, and the intel backlight driver doesn't work
correctly in this machine. If you are actually serious about the
mantra of "no user-space regressions", then for this machine at least,
you need to use the ACPI backlight with Windows8 OSI disabled, until
the intel backlight driver is fixed. My patch does that:

Felipe Contreras

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