Re: [PATCH] ACPI: Disable Windows 8 compatibility for some Lenovo ThinkPads

[Seth Forshee <seth.forshee@canonical.com>]

On Wed, Feb 13, 2013 at 08:55:58PM +0000, Matthew Garrett wrote:
> On Wed, Feb 13, 2013 at 02:32:28PM -0600, Seth Forshee wrote:
> > On Mon, Feb 11, 2013 at 07:09:14PM +0000, Matthew Garrett wrote:
> > > Right. My concern here is that Windows clearly doesn't trigger the 
> > > issue, and so there's some chance that we'll see similar issues on other 
> > > machines. Disabling Windows 8 compatibility isn't really an option. One 
> > > choice might be to have the ACPI video driver set all intermediate 
> > > values if the system makes the Windows 8 OSI call?
> > 
> > This turns out to have some problems. The hotkeys on the x230 at least
> > generate increase/decrease brightness notifications. In response
> > acpi_video reads the current brightness level via _BQC and decides what
> > the next value should be. A value adjacent to a working value is never
> > another working value on this machine, so _BCM does nothing. The next
> > time a notification comes _BQC returns the same value as it did the
> > previous time. Obviously this gets us nowhere.
> 
> Nrgh. Having this logic in the kernel has always been miserable. On the 
> other hand, having _BQC return wrong values is arguably even worse.
> 
> > The (untested) fix I've come up with is to use the cached value for the
> > current brightness rather than asking the firmware. I'm assuming though
> > that acpi_video would be using the cached values already if there wasn't
> > a chance that their changing without its knowledge?
> 
> Yeah. What I'd suggest here is calling _BQC after every change, and if 
> it returns the old value throw a firmware bug message and fall back to 
> using a cached one.

After every brightness change? Maybe it would be better to just test for
this during initialization, then set device->cap._BQC to 0 so we won't
use it.