fbdev: Track display blanking state

Store the display's blank status in struct fb_info.blank and track
it in fb_blank(). As an extra, the status is now available from the
sysfs blank attribute.

Support for blanking is optional. Therefore framebuffer_alloc()
initializes the state to FB_BLANK_UNBLANK (i.e., the display is
on). If the fb_blank callback has been set, register_framebuffer()
sets the state to FB_BLANK_POWERDOWN. On the first modeset, the
call to fb_blank() will update it to _UNBLANK. This is important,
as listeners to FB_EVENT_BLANK will now see the display being
switched on.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://lore.kernel.org/r/20250321095517.313713-3-tzimmermann@suse.de
Signed-off-by: Lee Jones <lee@kernel.org>

drivers/video/fbdev/core/fb_info.c

@@ -42,6 +42,7 @@ struct fb_info *framebuffer_alloc(size_t size, struct device *dev)
 
 	info->device = dev;
 	info->fbcon_rotate_hint = -1;
+	info->blank = FB_BLANK_UNBLANK;
 
 #if IS_ENABLED(CONFIG_FB_BACKLIGHT)
 	mutex_init(&info->bl_curve_mutex);

drivers/video/fbdev/core/fbmem.c

@@ -341,6 +341,7 @@ EXPORT_SYMBOL(fb_set_var);
 
 int fb_blank(struct fb_info *info, int blank)
 {
+	int old_blank = info->blank;
 	struct fb_event event;
 	int ret;
 
@@ -353,13 +354,19 @@ int fb_blank(struct fb_info *info, int blank)
 	event.info = info;
 	event.data = ␣
 
+	info->blank = blank;
+
 	ret = info->fbops->fb_blank(blank, info);
 	if (ret)
-		return ret;
+		goto err;
 
 	fb_notifier_call_chain(FB_EVENT_BLANK, &event);
 
 	return 0;
+
+err:
+	info->blank = old_blank;
+	return ret;
 }
 EXPORT_SYMBOL(fb_blank);
 
@@ -408,6 +415,14 @@ static int do_register_framebuffer(struct fb_info *fb_info)
 	mutex_init(&fb_info->lock);
 	mutex_init(&fb_info->mm_lock);
 
+	/*
+	 * With an fb_blank callback present, we assume that the
+	 * display is blank, so that fb_blank() enables it on the
+	 * first modeset.
+	 */
+	if (fb_info->fbops->fb_blank)
+		fb_info->blank = FB_BLANK_POWERDOWN;
+
 	fb_device_create(fb_info);
 
 	if (fb_info->pixmap.addr == NULL) {

drivers/video/fbdev/core/fbsysfs.c

@@ -242,11 +242,11 @@ static ssize_t store_blank(struct device *device,
 	return count;
 }
 
-static ssize_t show_blank(struct device *device,
-			  struct device_attribute *attr, char *buf)
+static ssize_t show_blank(struct device *device, struct device_attribute *attr, char *buf)
 {
-//	struct fb_info *fb_info = dev_get_drvdata(device);
-	return 0;
+	struct fb_info *fb_info = dev_get_drvdata(device);
+
+	return sysfs_emit(buf, "%d\n", fb_info->blank);
 }
 
 static ssize_t store_console(struct device *device,

include/linux/fb.h

@@ -472,6 +472,8 @@ struct fb_info {
 	struct list_head modelist;      /* mode list */
 	struct fb_videomode *mode;	/* current mode */
 
+	int blank; /* current blanking; see FB_BLANK_ constants */
+
 #if IS_ENABLED(CONFIG_FB_BACKLIGHT)
 	/* assigned backlight device */
 	/* set before framebuffer registration,