backlight: Replace fb events with a dedicated function call

Remove support for fb events from backlight subsystem. Provide the
helper backlight_notify_blank_all() instead. Also export the existing
helper backlight_notify_blank() to update a single backlight device.

In fbdev, call either helper to inform the backlight subsystem of
changes to a display's blank state. If the framebuffer device has a
specific backlight, only update this one; otherwise update all.

v4:
- protect blacklight declarations with IS_REACHABLE() (kernel test robot)
v3:
- declare empty fb_bl_notify_blank() as static inline (kernel test robot)

Signed-off-by: Thomas Zimmermann <[e-mail]>
Acked-by: Simona Vetter <[e-mail]>
Reviewed-by: "Daniel Thompson (RISCstar)" <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Lee Jones <[e-mail]>

drivers/video/backlight/backlight.c

···
 #include <linux/notifier.h>
 #include <linux/ctype.h>
 #include <linux/err.h>
-#include <linux/fb.h>
 #include <linux/slab.h>
 
 #ifdef CONFIG_PMAC_BACKLIGHT
···
  * a hot-key to adjust backlight, the driver must notify the backlight
  * core that brightness has changed using backlight_force_update().
  *
- * The backlight driver core receives notifications from fbdev and
- * if the event is FB_EVENT_BLANK and if the value of blank, from the
- * FBIOBLANK ioctrl, results in a change in the backlight state the
- * update_status() operation is called.
+ * Display drives can control the backlight device's status using
+ * backlight_notify_blank() and backlight_notify_blank_all(). If this
+ * results in a change in the backlight state the functions call the
+ * update_status() operation.
  */
 
 static struct list_head backlight_dev_list;
···
 	[BACKLIGHT_SCALE_NON_LINEAR]	= "non-linear",
 };
 
-#if defined(CONFIG_FB_CORE) || (defined(CONFIG_FB_CORE_MODULE) && \
-				defined(CONFIG_BACKLIGHT_CLASS_DEVICE_MODULE))
-static void backlight_notify_blank(struct backlight_device *bd,
-				   struct device *display_dev,
-				   bool fb_on, bool prev_fb_on)
+void backlight_notify_blank(struct backlight_device *bd, struct device *display_dev,
+			    bool fb_on, bool prev_fb_on)
 {
 	guard(mutex)(&bd->ops_lock);
 
···
 		}
 	}
 }
+EXPORT_SYMBOL(backlight_notify_blank);
 
-/*
- * fb_notifier_callback
- *
- * This callback gets called when something important happens inside a
- * framebuffer driver. The backlight core only cares about FB_BLANK_UNBLANK
- * which is reported to the driver using backlight_update_status()
- * as a state change.
- *
- * There may be several fbdev's connected to the backlight device,
- * in which case they are kept track of. A state change is only reported
- * if there is a change in backlight for the specified fbdev.
- */
-static int fb_notifier_callback(struct notifier_block *self,
-				unsigned long event, void *data)
+void backlight_notify_blank_all(struct device *display_dev, bool fb_on, bool prev_fb_on)
 {
 	struct backlight_device *bd;
-	struct fb_event *evdata = data;
-	struct fb_info *info = evdata->info;
-	const int *fb_blank = evdata->data;
-	struct backlight_device *fb_bd = fb_bl_device(info);
-	bool fb_on, prev_fb_on;
 
-	/* If we aren't interested in this event, skip it immediately ... */
-	if (event != FB_EVENT_BLANK)
-		return 0;
+	guard(mutex)(&backlight_dev_list_mutex);
 
-	bd = container_of(self, struct backlight_device, fb_notif);
-
-	if (fb_bd && fb_bd != bd)
-		return 0;
-
-	fb_on = fb_blank[0] == FB_BLANK_UNBLANK;
-	prev_fb_on = fb_blank[1] == FB_BLANK_UNBLANK;
-
-	backlight_notify_blank(bd, info->device, fb_on, prev_fb_on);
-
-	return 0;
+	list_for_each_entry(bd, &backlight_dev_list, entry)
+		backlight_notify_blank(bd, display_dev, fb_on, prev_fb_on);
 }
-
-static int backlight_register_fb(struct backlight_device *bd)
-{
-	memset(&bd->fb_notif, 0, sizeof(bd->fb_notif));
-	bd->fb_notif.notifier_call = fb_notifier_callback;
-
-	return fb_register_client(&bd->fb_notif);
-}
-
-static void backlight_unregister_fb(struct backlight_device *bd)
-{
-	fb_unregister_client(&bd->fb_notif);
-}
-#else
-static inline int backlight_register_fb(struct backlight_device *bd)
-{
-	return 0;
-}
-
-static inline void backlight_unregister_fb(struct backlight_device *bd)
-{
-}
-#endif /* CONFIG_FB_CORE */
+EXPORT_SYMBOL(backlight_notify_blank_all);
 
 static void backlight_generate_event(struct backlight_device *bd,
 				     enum backlight_update_reason reason)
···
 		return ERR_PTR(rc);
 	}
 
-	rc = backlight_register_fb(new_bd);
-	if (rc) {
-		device_unregister(&new_bd->dev);
-		return ERR_PTR(rc);
-	}
-
 	new_bd->ops = ops;
 
 #ifdef CONFIG_PMAC_BACKLIGHT
···
 	bd->ops = NULL;
 	mutex_unlock(&bd->ops_lock);
 
-	backlight_unregister_fb(bd);
 	device_unregister(&bd->dev);
 }
 EXPORT_SYMBOL(backlight_device_unregister);

drivers/video/fbdev/core/fb_backlight.c

···
 // SPDX-License-Identifier: GPL-2.0-or-later
 
+#include <linux/backlight.h>
 #include <linux/export.h>
 #include <linux/fb.h>
 #include <linux/mutex.h>
···
 	return info->bl_dev;
 }
 EXPORT_SYMBOL(fb_bl_device);
+
+void fb_bl_notify_blank(struct fb_info *info, int old_blank)
+{
+	bool on = info->blank == FB_BLANK_UNBLANK;
+	bool prev_on = old_blank == FB_BLANK_UNBLANK;
+
+	if (info->bl_dev)
+		backlight_notify_blank(info->bl_dev, info->device, on, prev_on);
+	else
+		backlight_notify_blank_all(info->device, on, prev_on);
+}
 #endif

drivers/video/fbdev/core/fbmem.c

···
 	if (ret)
 		goto err;
 
+	fb_bl_notify_blank(info, old_blank);
+
 	fb_notifier_call_chain(FB_EVENT_BLANK, &event);
 
 	return 0;

include/linux/backlight.h

···
 #include <linux/device.h>
 #include <linux/fb.h>
 #include <linux/mutex.h>
-#include <linux/notifier.h>
 #include <linux/types.h>
 
 /**
···
 	const struct backlight_ops *ops;
 
 	/**
-	 * @fb_notif: The framebuffer notifier block
-	 */
-	struct notifier_block fb_notif;
-
-	/**
 	 * @entry: List entry of all registered backlight devices
 	 */
 	struct list_head entry;
···
 struct backlight_device *backlight_device_get_by_type(enum backlight_type type);
 int backlight_device_set_brightness(struct backlight_device *bd,
 				    unsigned long brightness);
+
+#if IS_REACHABLE(CONFIG_BACKLIGHT_CLASS_DEVICE)
+void backlight_notify_blank(struct backlight_device *bd,
+			    struct device *display_dev,
+			    bool fb_on, bool prev_fb_on);
+void backlight_notify_blank_all(struct device *display_dev,
+				bool fb_on, bool prev_fb_on);
+#else
+static inline void backlight_notify_blank(struct backlight_device *bd,
+					  struct device *display_dev,
+					  bool fb_on, bool prev_fb_on)
+{ }
+static inline void backlight_notify_blank_all(struct device *display_dev,
+					      bool fb_on, bool prev_fb_on)
+{ }
+#endif
 
 #define to_backlight_device(obj) container_of(obj, struct backlight_device, dev)
 

include/linux/fb.h

···
 
 #if IS_ENABLED(CONFIG_FB_BACKLIGHT)
 struct backlight_device *fb_bl_device(struct fb_info *info);
+void fb_bl_notify_blank(struct fb_info *info, int old_blank);
 #else
 static inline struct backlight_device *fb_bl_device(struct fb_info *info)
 {
 	return NULL;
 }
+
+static inline void fb_bl_notify_blank(struct fb_info *info, int old_blank)
+{ }
 #endif
 
 static inline struct lcd_device *fb_lcd_device(struct fb_info *info)