backlight: backlight: Introduce backlight_get_brightness()

Based on an idea from Emil Velikov, add a helper that checks
backlight_is_blank() and return 0 as brightness if display is blank or
the property value if not.

This allows us to simplify the update_status() implementation
in most of the backlight drivers.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

include/linux/backlight.h

@@ -417,6 +417,25 @@ static inline bool backlight_is_blank(const struct backlight_device *bd)
 	       bd->props.state & (BL_CORE_SUSPENDED | BL_CORE_FBBLANK);
 }
 
+/**
+ * backlight_get_brightness - Returns the current brightness value
+ * @bd: the backlight device
+ *
+ * Returns the current brightness value, taking in consideration the current
+ * state. If backlight_is_blank() returns true then return 0 as brightness
+ * otherwise return the current brightness property value.
+ *
+ * Backlight drivers are expected to use this function in their update_status()
+ * operation to get the brightness value.
+ */
+static inline int backlight_get_brightness(const struct backlight_device *bd)
+{
+	if (backlight_is_blank(bd))
+		return 0;
+	else
+		return bd->props.brightness;
+}
+
 struct backlight_device *
 backlight_device_register(const char *name, struct device *dev, void *devdata,
 			  const struct backlight_ops *ops,