[PATCH v2] nouveau: Don't check acpi_video_backlight_support() before registering backlight

[Hans de Goede]

Hi,

On 05/12/2014 09:32 AM, Hans de Goede wrote:
> acpi_video_backlight_support() is supposed to be called by other (vendor
> specific) firmware backlight controls, not by native / raw backlight controls
> like nv_backlight.
> 
> Userspace will normally prefer firmware interfaces over raw interfaces, so
> if acpi_video backlight support is present it will use that even if
> nv_backlight is registered as well.
> 
> Except when video.use_native_backlight is present on the kernel cmdline
> (or enabled through a dmi based quirk). As the name indicates the goal here
> is to make only the raw interface available to userspace so that it will use
> that (it only does this when it sees a win8 compliant bios).
> 
> This is done by:
> 1) Not registering any acpi_video# backlight devices; and
> 2) Making acpi_video_backlight_support() return true so that other firmware
> drivers, ie acer_wmi, thinkpad_acpi, dell_laptop, etc. Don't register their
> own vender specific interfaces.
> 
> Currently nouveau breaks this setup, as when acpi_video_backlight_support()
> returns true, it does not register itself, resulting in no backlight control
> at all.
> 
> This is esp. going to be a problem with 3.16 which will default to
> video.use_native_backlight=1, and thus nouveau based laptops with a win8 bios
> will get no backlight control at all.
> 
> This also likely explains why the previous attempt to make
> video.use_native_backlight=1 the default was not a success, as without this
> patch having a default of video.use_native_backlight=1 will cause regressions.
> 
> Note this effectively reverts commit 5bead799
> 
> Also see: https://bugzilla.redhat.com/show_bug.cgi?id=1093171
> 
> Signed-off-by: Hans de Goede <hdegoede at redhat.com>

Ignore the v2 in the subject please it is bogus.

Question should this also go to stable ? I guess that for old / broken userspace
it may lead to userspace doing the wrong thing in some cases. So that goes
against adding it to stable. OTOH the intel driver has been always registering
its native backlight interface for ages, so userspace should know better; and
this fix is needed (together with a quirk to set video.use_native_backlight=1
by default), to get the backlight control to work on the ThinkPad W530.

Regards,

Hans