ACPI: video: Add acpi_video_backlight_use_native() helper

ATM on x86 laptops where we want userspace to use the acpi_video backlight
device we often register both the GPU's native backlight device and
acpi_video's firmware acpi_video# backlight device. This relies on
userspace preferring firmware type backlight devices over native ones, but
registering 2 backlight devices for a single display really is undesirable.

On x86 laptops where the native GPU backlight device should be used,
the registering of other backlight devices is avoided by their drivers
using acpi_video_get_backlight_type() and only registering their backlight
if the return value matches their type.

acpi_video_get_backlight_type() uses
backlight_device_get_by_type(BACKLIGHT_RAW) to determine if a native
driver is available and will never return native if this returns
false. This means that the GPU's native backlight registering code
cannot just call acpi_video_get_backlight_type() to determine if it
should register its backlight, since acpi_video_get_backlight_type() will
never return native until the native backlight has already registered.

To fix this add a new internal native function parameter to
acpi_video_get_backlight_type(), which when set to true will make
acpi_video_get_backlight_type() behave as if a native backlight has
already been registered.

And add a new acpi_video_backlight_use_native() helper, which sets this
to true, for use in native GPU backlight code.

Changes in v2:
- Replace adding a native parameter to acpi_video_get_backlight_type() with
  adding a new acpi_video_backlight_use_native() helper.
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

drivers/acpi/video_detect.c

@@ -17,8 +17,9 @@
  * Otherwise vendor specific drivers like thinkpad_acpi, asus-laptop,
  * sony_acpi,... can take care about backlight brightness.
  *
- * Backlight drivers can use acpi_video_get_backlight_type() to determine
- * which driver should handle the backlight.
+ * Backlight drivers can use acpi_video_get_backlight_type() to determine which
+ * driver should handle the backlight. RAW/GPU-driver backlight drivers must
+ * use the acpi_video_backlight_use_native() helper for this.
  *
  * If CONFIG_ACPI_VIDEO is neither set as "compiled in" (y) nor as a module (m)
  * this file will not be compiled and acpi_video_get_backlight_type() will
@@ -571,9 +572,10 @@ static int acpi_video_backlight_notify(struct notifier_block *nb,
  * Arguably the native on win8 check should be done first, but that would
  * be a behavior change, which may causes issues.
  */
-enum acpi_backlight_type acpi_video_get_backlight_type(void)
+static enum acpi_backlight_type __acpi_video_get_backlight_type(bool native)
 {
 	static DEFINE_MUTEX(init_mutex);
+	static bool native_available;
 	static bool init_done;
 	static long video_caps;
 
@@ -593,6 +595,8 @@ enum acpi_backlight_type acpi_video_get_backlight_type(void)
 			backlight_notifier_registered = true;
 		init_done = true;
 	}
+	if (native)
+		native_available = true;
 	mutex_unlock(&init_mutex);
 
 	if (acpi_backlight_cmdline != acpi_backlight_undef)
@@ -604,13 +608,25 @@ enum acpi_backlight_type acpi_video_get_backlight_type(void)
 	if (!(video_caps & ACPI_VIDEO_BACKLIGHT))
 		return acpi_backlight_vendor;
 
-	if (acpi_osi_is_win8() && backlight_device_get_by_type(BACKLIGHT_RAW))
+	if (acpi_osi_is_win8() &&
+	    (native_available || backlight_device_get_by_type(BACKLIGHT_RAW)))
 		return acpi_backlight_native;
 
 	return acpi_backlight_video;
 }
+
+enum acpi_backlight_type acpi_video_get_backlight_type(void)
+{
+	return __acpi_video_get_backlight_type(false);
+}
 EXPORT_SYMBOL(acpi_video_get_backlight_type);
 
+bool acpi_video_backlight_use_native(void)
+{
+	return __acpi_video_get_backlight_type(true) == acpi_backlight_native;
+}
+EXPORT_SYMBOL(acpi_video_backlight_use_native);
+
 /*
  * Set the preferred backlight interface type based on DMI info.
  * This function allows DMI blacklists to be implemented by external

include/acpi/video.h

@@ -56,6 +56,7 @@ extern void acpi_video_unregister(void);
 extern int acpi_video_get_edid(struct acpi_device *device, int type,
 			       int device_id, void **edid);
 extern enum acpi_backlight_type acpi_video_get_backlight_type(void);
+extern bool acpi_video_backlight_use_native(void);
 extern void acpi_video_set_dmi_backlight_type(enum acpi_backlight_type type);
 /*
  * Note: The value returned by acpi_video_handles_brightness_key_presses()
@@ -77,6 +78,10 @@ static inline enum acpi_backlight_type acpi_video_get_backlight_type(void)
 {
 	return acpi_backlight_vendor;
 }
+static inline bool acpi_video_backlight_use_native(void)
+{
+	return true;
+}
 static inline void acpi_video_set_dmi_backlight_type(enum acpi_backlight_type type)
 {
 }