[Nouveau] nouveau exposes backlight controls in presence of ACPI

[Aaron Sowry]

> 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.

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
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