backlight: update backlight status when necessary

We don't have to update a backlight status every time a blanking or
unblanking event comes because the backlight status may have already
been what we want.  Another thought is that one backlight device may be
shared by multiple framebuffers.  We don't hope blanking one of the
framebuffers may turn the backlight off for all the other framebuffers,
since they are likely being active to display something.

This patch makes the backlight status be updated only when the relevant
backlight device's use count changes from zero to one or from one to
zero.

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
Cc: Jingoo Han <jg1.han@samsung.com>
Cc: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/video/backlight/backlight.c

@@ -34,7 +34,7 @@ static const char *const backlight_types[] = {
 			   defined(CONFIG_BACKLIGHT_CLASS_DEVICE_MODULE))
 /* This callback gets called when something important happens inside a
  * framebuffer driver. We're looking if that important event is blanking,
- * and if it is, we're switching backlight power as well ...
+ * and if it is and necessary, we're switching backlight power as well ...
  */
 static int fb_notifier_callback(struct notifier_block *self,
 				unsigned long event, void *data)
@@ -60,6 +60,7 @@ static int fb_notifier_callback(struct notifier_block *self,
 				if (!bd->use_count++) {
 					bd->props.state &= ~BL_CORE_FBBLANK;
 					bd->props.fb_blank = FB_BLANK_UNBLANK;
+					backlight_update_status(bd);
 				}
 			} else if (fb_blank != FB_BLANK_UNBLANK &&
 				   bd->fb_bl_on[node]) {
@@ -67,9 +68,9 @@ static int fb_notifier_callback(struct notifier_block *self,
 				if (!(--bd->use_count)) {
 					bd->props.state |= BL_CORE_FBBLANK;
 					bd->props.fb_blank = fb_blank;
+					backlight_update_status(bd);
 				}
 			}
-			backlight_update_status(bd);
 		}
 	mutex_unlock(&bd->ops_lock);
 	return 0;