[Nouveau] nouveau exposes backlight controls in presence of ACPI

[Ben Skeggs]

On Tue, 2010-11-02 at 15:51 +0100, Aaron Sowry wrote:
> > However.  There's also the platform-specific modules (thinkpad etc) that
> > will provide their own backlight methods if the standard ACPI mechanism
> > isn't available.  I didn't see any immediately obvious way of knowing
> > whether or not they were being provided.
> > 
> > I'm not too certain the best way to deal with this, any ideas? :)
> 
> See attached patch, which prevents nouveau registering a backlight control if ACPI already provides one. If there are no glaring errors in the code/implementation then I would like to submit it.
This is much like what I intended to commit.  I guess it doesn't hurt to
do so, it doesn't help with all cases yet but if it fixes issues for
some people, why not.

Ben.
> 
> Dealing with platform-specific modules is tricky because it looks like nouveau initialises itself quite early on in the boot process:
> 
> [aaron at jules ~]$ dmesg | grep ACPI
> ...
> [    3.359377] nouveau 0000:01:00.0: power state changed by ACPI to D0
> [    3.359560] nouveau 0000:01:00.0: power state changed by ACPI to D0
> [    4.860678] [drm] nouveau 0000:01:00.0: Detected ACPI backlight support, not registering control
> [   14.206997] ACPI: WMI: Skipping duplicate GUID 05901221-D566-11D1-B2F0-00A0C9062910
> [   14.207693] ACPI: WMI: Mapper loaded
> [   17.686907] thinkpad_acpi: ThinkPad ACPI Extras v0.24
> [   17.689517] thinkpad_acpi: This ThinkPad has standard ACPI backlight brightness control, supported by the ACPI video driver
> [   17.756039] thinkpad_acpi: Standard ACPI backlight interface available, not loading native one.
> 
> My suggestion would be that it is the responsibility of user-space to select an appropriate ACPI backlight control.
> 
> /Aaron