media: mc-entity: Add a new helper function to get a remote pad for a pad

The newly added media_entity_remote_source_pad_unique() helper function
handles use cases where the entity has a link enabled uniqueness
constraint covering all pads. There are use cases where the constraint
covers a specific pad only. Add a new media_pad_remote_pad_unique()
function to handle this. It operates as
media_entity_remote_source_pad_unique(), but on a given pad instead of
on the entity.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

Documentation/driver-api/media/mc-core.rst

@@ -186,8 +186,9 @@ is required and the graph structure can be freed normally.
 
 Helper functions can be used to find a link between two given pads, or a pad
 connected to another pad through an enabled link
-(:c:func:`media_entity_find_link()`, :c:func:`media_pad_remote_pad_first()` and
+(:c:func:`media_entity_find_link()`, :c:func:`media_pad_remote_pad_first()`n
-:c:func:`media_entity_remote_source_pad_unique()`).
+:c:func:`media_entity_remote_source_pad_unique()` and
+:c:func:`media_pad_remote_pad_unique()`).
 
 Use count and power handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

drivers/media/mc/mc-entity.c

@@ -960,6 +960,37 @@ media_entity_remote_pad_unique(const struct media_entity *entity,
 }
 EXPORT_SYMBOL_GPL(media_entity_remote_pad_unique);
 
+struct media_pad *media_pad_remote_pad_unique(const struct media_pad *pad)
+{
+	struct media_pad *found_pad = NULL;
+	struct media_link *link;
+
+	list_for_each_entry(link, &pad->entity->links, list) {
+		struct media_pad *remote_pad;
+
+		if (!(link->flags & MEDIA_LNK_FL_ENABLED))
+			continue;
+
+		if (link->sink == pad)
+			remote_pad = link->source;
+		else if (link->source == pad)
+			remote_pad = link->sink;
+		else
+			continue;
+
+		if (found_pad)
+			return ERR_PTR(-ENOTUNIQ);
+
+		found_pad = remote_pad;
+	}
+
+	if (!found_pad)
+		return ERR_PTR(-ENOLINK);
+
+	return found_pad;
+}
+EXPORT_SYMBOL_GPL(media_pad_remote_pad_unique);
+
 static void media_interface_init(struct media_device *mdev,
 				 struct media_interface *intf,
 				 u32 gobj_type,

include/media/media-entity.h

@@ -859,6 +859,24 @@ struct media_link *media_entity_find_link(struct media_pad *source,
  */
 struct media_pad *media_pad_remote_pad_first(const struct media_pad *pad);
 
+/**
+ * media_pad_remote_pad_unique - Find a remote pad connected to a pad
+ * @pad: The pad
+ *
+ * Search for and return a remote pad connected to @pad through an enabled
+ * link. If multiple (or no) remote pads are found, an error is returned.
+ *
+ * The uniqueness constraint makes this helper function suitable for entities
+ * that support a single active source at a time on a given pad.
+ *
+ * Return: A pointer to the remote pad, or one of the following error pointers
+ * if an error occurs:
+ *
+ * * -ENOTUNIQ - Multiple links are enabled
+ * * -ENOLINK - No connected pad found
+ */
+struct media_pad *media_pad_remote_pad_unique(const struct media_pad *pad);
+
 /**
  * media_entity_remote_pad_unique - Find a remote pad connected to an entity
  * @entity: The entity