backlight: Match backlight device against struct fb_info.bl_dev

Framebuffer drivers for devices with dedicated backlight are supposed
to set struct fb_info.bl_dev to the backlight's respective device. Use
the value to match backlight and framebuffer in the backlight core code.

The code first tests against struct backlight_ops.check_ops. If this
test succeeds, it performs the test against fbdev. So backlight drivers
can override the later test as before.

Fbdev's backlight support depends on CONFIG_FB_BACKLIGHT. To avoid
ifdef in the code, the new helper fb_bl_device() returns the backlight
device, or NULL if the config option has been disabled. The test in
the backlight code will then do nothing.

v4:
	* declare empty fb_bl_device() as static inline
	* export fb_bl_device()
v3:
	* hide ifdef in fb_bl_device() (Lee)
	* no if-else blocks (Andy)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20240305162425.23845-2-tzimmermann@suse.deSigned-off-by: Lee Jones <lee@kernel.org>

drivers/video/backlight/backlight.c

@@ -98,7 +98,9 @@ static int fb_notifier_callback(struct notifier_block *self,
 {
 	struct backlight_device *bd;
 	struct fb_event *evdata = data;
-	int node = evdata->info->node;
+	struct fb_info *info = evdata->info;
+	struct backlight_device *fb_bd = fb_bl_device(info);
+	int node = info->node;
 	int fb_blank = 0;
 
 	/* If we aren't interested in this event, skip it immediately ... */
@@ -110,7 +112,9 @@ static int fb_notifier_callback(struct notifier_block *self,
 
 	if (!bd->ops)
 		goto out;
-	if (bd->ops->check_fb && !bd->ops->check_fb(bd, evdata->info))
+	if (bd->ops->check_fb && !bd->ops->check_fb(bd, info))
+		goto out;
+	if (fb_bd && fb_bd != bd)
 		goto out;
 
 	fb_blank = *(int *)evdata->data;

drivers/video/fbdev/core/fb_backlight.c

@@ -30,4 +30,10 @@ void fb_bl_default_curve(struct fb_info *fb_info, u8 off, u8 min, u8 max)
 	mutex_unlock(&fb_info->bl_curve_mutex);
 }
 EXPORT_SYMBOL_GPL(fb_bl_default_curve);
+
+struct backlight_device *fb_bl_device(struct fb_info *info)
+{
+	return info->bl_dev;
+}
+EXPORT_SYMBOL(fb_bl_device);
 #endif

include/linux/fb.h

@@ -738,6 +738,15 @@ extern struct fb_info *framebuffer_alloc(size_t size, struct device *dev);
 extern void framebuffer_release(struct fb_info *info);
 extern void fb_bl_default_curve(struct fb_info *fb_info, u8 off, u8 min, u8 max);
 
+#if IS_ENABLED(CONFIG_FB_BACKLIGHT)
+struct backlight_device *fb_bl_device(struct fb_info *info);
+#else
+static inline struct backlight_device *fb_bl_device(struct fb_info *info)
+{
+	return NULL;
+}
+#endif
+
 /* fbmon.c */
 #define FB_MAXTIMINGS		0
 #define FB_VSYNCTIMINGS		1