ACPI / video: Add a acpi_video_handles_brightness_key_presses() helper

Several drivers want to know if the acpi-video is generating key-presses
for brightness change hotkeys to avoid sending double key-events to
userspace for these. Currently these driver use this construct for this:

	if (acpi_video_get_backlight_type() == acpi_backlight_vendor)
		report_brightness_key_event();

This indirect way of detecting if acpi-video is active does not make the
code easier to understand, and in some cases it is wrong because just
because the preferred type != vendor does not mean that acpi-video is
actually listening for brightness events, e.g. there may be no acpi-video
bus on the system at all.

This commit adds a acpi_video_handles_brightness_key_presses() helper
function, making the code needing this functionality both easier to read
and more correct.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/acpi_video.c

@@ -2072,6 +2072,18 @@ void acpi_video_unregister_backlight(void)
 	mutex_unlock(&register_count_mutex);
 }
 
+bool acpi_video_handles_brightness_key_presses(void)
+{
+	bool have_video_busses;
+
+	mutex_lock(&video_list_lock);
+	have_video_busses = !list_empty(&video_bus_head);
+	mutex_unlock(&video_list_lock);
+
+	return have_video_busses;
+}
+EXPORT_SYMBOL(acpi_video_handles_brightness_key_presses);
+
 /*
  * This is kind of nasty. Hardware using Intel chipsets may require
  * the video opregion code to be run first in order to initialise

include/acpi/video.h

@@ -2,6 +2,7 @@
 #define __ACPI_VIDEO_H
 
 #include <linux/errno.h> /* for ENODEV */
+#include <linux/types.h> /* for bool */
 
 struct acpi_device;
 
@@ -31,6 +32,7 @@ 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 void acpi_video_set_dmi_backlight_type(enum acpi_backlight_type type);
+extern bool acpi_video_handles_brightness_key_presses(void);
 #else
 static inline int acpi_video_register(void) { return 0; }
 static inline void acpi_video_unregister(void) { return; }
@@ -46,6 +48,10 @@ static inline enum acpi_backlight_type acpi_video_get_backlight_type(void)
 static inline void acpi_video_set_dmi_backlight_type(enum acpi_backlight_type type)
 {
 }
+static inline bool acpi_video_handles_brightness_key_presses(void)
+{
+	return false;
+}
 #endif
 
 #endif