ACPI: video: Prefer native over vendor

When available prefer native backlight control over vendor backlight
control.

Testing has shown that there are quite a few laptop models which rely
on native backlight control (they don't have ACPI video bus backlight
control) and on which acpi_osi_is_win8() returns false.

Currently __acpi_video_get_backlight_type() returns vendor on these
laptops, leading to an empty /sys/class/backlight.

As a workaround for this acpi_video_backlight_use_native() has been
temporarily changed to always return true.

This re-introduces the problem of having multiple backlight
devices under /sys/class/backlight for a single panel.

Change __acpi_video_get_backlight_type() to prefer native over vendor
when available. So that it returns native on these models.

And change acpi_video_backlight_use_native() back to only return
true when __acpi_video_get_backlight_type() returns native.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/video_detect.c

@@ -776,10 +776,10 @@ static enum acpi_backlight_type __acpi_video_get_backlight_type(bool native)
 		return acpi_backlight_video;
 
 	/* Use native if available */
-	if (native_available && prefer_native_over_acpi_video())
+	if (native_available)
 		return acpi_backlight_native;
 
-	/* No ACPI video (old hw), use vendor specific fw methods. */
+	/* No ACPI video/native (old hw), use vendor specific fw methods. */
 	return acpi_backlight_vendor;
 }
 
@@ -791,18 +791,6 @@ EXPORT_SYMBOL(acpi_video_get_backlight_type);
 
 bool acpi_video_backlight_use_native(void)
 {
-	/*
-	 * Call __acpi_video_get_backlight_type() to let it know that
-	 * a native backlight is available.
-	 */
-	__acpi_video_get_backlight_type(true);
-
-	/*
-	 * For now just always return true. There is a whole bunch of laptop
-	 * models where (video_caps & ACPI_VIDEO_BACKLIGHT) is false causing
-	 * __acpi_video_get_backlight_type() to return vendor, while these
-	 * models only have a native backlight control.
-	 */
-	return true;
+	return __acpi_video_get_backlight_type(true) == acpi_backlight_native;
 }
 EXPORT_SYMBOL(acpi_video_backlight_use_native);