media: v4l2-async: Create links during v4l2_async_match_notify()

Upon an async fwnode match, there's some typical behaviour that the
notifier and matching subdev will want to do. For example, a notifier
representing a sensor matching to an async subdev representing its
VCM will want to create an ancillary link to expose that relationship
to userspace.

To avoid lots of code in individual drivers, try to build these links
within v4l2 core.

Signed-off-by: Daniel Scally <djrscally@gmail.com>

drivers/media/v4l2-core/v4l2-async.c

@@ -275,6 +275,45 @@ v4l2_async_nf_try_complete(struct v4l2_async_notifier *notifier)
 static int
 v4l2_async_nf_try_all_subdevs(struct v4l2_async_notifier *notifier);
 
+static int
+__v4l2_async_create_ancillary_link(struct v4l2_async_notifier *notifier,
+				   struct v4l2_subdev *sd)
+{
+	struct media_link *link;
+
+	if (sd->entity.function != MEDIA_ENT_F_LENS &&
+	    sd->entity.function != MEDIA_ENT_F_FLASH)
+		return -EINVAL;
+
+	link = media_create_ancillary_link(&notifier->sd->entity, &sd->entity,
+					   MEDIA_LNK_FL_ENABLED |
+					   MEDIA_LNK_FL_IMMUTABLE);
+
+	return IS_ERR(link) ? PTR_ERR(link) : 0;
+}
+
+/*
+ * Setup links on behalf of the notifier and subdev, where it's obvious what
+ * should be done. At the moment, we only support cases where the notifier
+ * is a sensor and the subdev is a lens.
+ *
+ * TODO: Setup pad links if the notifier's function is MEDIA_ENT_F_VID_IF_BRIDGE
+ * and the subdev's is MEDIA_ENT_F_CAM_SENSOR
+ */
+static int v4l2_async_try_create_links(struct v4l2_async_notifier *notifier,
+				       struct v4l2_subdev *sd)
+{
+	if (!notifier->sd)
+		return 0;
+
+	switch (notifier->sd->entity.function) {
+	case MEDIA_ENT_F_CAM_SENSOR:
+		return __v4l2_async_create_ancillary_link(notifier, sd);
+	default:
+		return 0;
+	}
+}
+
 static int v4l2_async_match_notify(struct v4l2_async_notifier *notifier,
 				   struct v4l2_device *v4l2_dev,
 				   struct v4l2_subdev *sd,
@@ -293,6 +332,18 @@ static int v4l2_async_match_notify(struct v4l2_async_notifier *notifier,
 		return ret;
 	}
 
+	/*
+	 * Depending of the function of the entities involved, we may want to
+	 * create links between them (for example between a sensor and its lens
+	 * or between a sensor's source pad and the connected device's sink
+	 * pad)
+	 */
+	ret = v4l2_async_try_create_links(notifier, sd);
+	if (ret) {
+		v4l2_device_unregister_subdev(sd);
+		return ret;
+	}
+
 	/* Remove from the waiting list */
 	list_del(&asd->list);
 	sd->asd = asd;