[media] media: Add a function removing all links of a media entity

This function allows to remove all media entity's links to other
entities, leaving no references to a media entity's links array
at its remote entities.
Currently, when a driver of some entity is removed it will free its
media entities links[] array, leaving dangling pointers at other
entities that are part of same media graph. This is troublesome when
drivers of a media device entities are in separate kernel modules,
removing only some modules will leave others in an incorrect state.
This function is intended to be used when an entity is being
unregistered from a media device.
With an assumption that normally the media links should be created
between media entities registered to a media device, with the graph
mutex held.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/media-entity.c

@@ -429,6 +429,56 @@ media_entity_create_link(struct media_entity *source, u16 source_pad,
 }
 EXPORT_SYMBOL_GPL(media_entity_create_link);
 
+void __media_entity_remove_links(struct media_entity *entity)
+{
+	unsigned int i;
+
+	for (i = 0; i < entity->num_links; i++) {
+		struct media_link *link = &entity->links[i];
+		struct media_entity *remote;
+		unsigned int r = 0;
+
+		if (link->source->entity == entity)
+			remote = link->sink->entity;
+		else
+			remote = link->source->entity;
+
+		while (r < remote->num_links) {
+			struct media_link *rlink = &remote->links[r];
+
+			if (rlink != link->reverse) {
+				r++;
+				continue;
+			}
+
+			if (link->source->entity == entity)
+				remote->num_backlinks--;
+
+			if (--remote->num_links == 0)
+				break;
+
+			/* Insert last entry in place of the dropped link. */
+			*rlink = remote->links[remote->num_links];
+		}
+	}
+
+	entity->num_links = 0;
+	entity->num_backlinks = 0;
+}
+EXPORT_SYMBOL_GPL(__media_entity_remove_links);
+
+void media_entity_remove_links(struct media_entity *entity)
+{
+	/* Do nothing if the entity is not registered. */
+	if (entity->parent == NULL)
+		return;
+
+	mutex_lock(&entity->parent->graph_mutex);
+	__media_entity_remove_links(entity);
+	mutex_unlock(&entity->parent->graph_mutex);
+}
+EXPORT_SYMBOL_GPL(media_entity_remove_links);
+
 static int __media_entity_setup_link_notify(struct media_link *link, u32 flags)
 {
 	int ret;