Currently the acpi video module export the backlight interface to sysfs
also if acpi_video_device_lcd_query_levels() fails to read _BLC method
(e.g.  because the method is not available).  In this case the userspace
don't know which brightness level are supported and can't set a brightness
level (echo return with: "write error: Invalid Argument").  This happend
e.g.  on a ASUS RF1 (correct supported by the asus-laptop module).

The video module should not export the backlight interface if query _BLC fail,
because you can't set anything from userspace and this make it useless.

http://bugzilla.kernel.org/show_bug.cgi?id=8375Signed-off-by: Danny Kukawka <dkukawka@suse.de>
Acked-by: Luming Yu <luming.yu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>