media: media.h: Add new media link type

To describe in the kernel the connection between devices and their
supporting peripherals (for example, a camera sensor and the vcm
driving the focusing lens for it), add a new type of media link
to introduce the concept of these ancillary links.

Add some elements to the uAPI documentation to explain the new link
type, their purpose and some aspects of their current implementation.
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Scally <djrscally@gmail.com>
Reviewed-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

Documentation/userspace-api/media/mediactl/media-controller-model.rst

@@ -33,3 +33,9 @@ are:
 
 -  An **interface link** is a point-to-point bidirectional control
    connection between a Linux Kernel interface and an entity.
+
+- An **ancillary link** is a point-to-point connection denoting that two
+  entities form a single logical unit. For example this could represent the
+  fact that a particular camera sensor and lens controller form a single
+  physical module, meaning this lens controller drives the lens for this
+  camera sensor.
\ No newline at end of file

Documentation/userspace-api/media/mediactl/media-types.rst

@@ -412,14 +412,21 @@ must be set for every pad.
 	  is set by drivers and is read-only for applications.
 
     *  -  ``MEDIA_LNK_FL_LINK_TYPE``
-       -  This is a bitmask that defines the type of the link. Currently,
-	  two types of links are supported:
+       -  This is a bitmask that defines the type of the link. The following
+	  link types are currently supported:
 
 	  .. _MEDIA-LNK-FL-DATA-LINK:
 
-	  ``MEDIA_LNK_FL_DATA_LINK`` if the link is between two pads
+	  ``MEDIA_LNK_FL_DATA_LINK`` for links that represent a data connection
+	  between two pads.
 
 	  .. _MEDIA-LNK-FL-INTERFACE-LINK:
 
-	  ``MEDIA_LNK_FL_INTERFACE_LINK`` if the link is between an
-	  interface and an entity
+	  ``MEDIA_LNK_FL_INTERFACE_LINK`` for links that associate an entity to its
+	  interface.
+
+	  .. _MEDIA-LNK-FL-ANCILLARY-LINK:
+
+	  ``MEDIA_LNK_FL_ANCILLARY_LINK`` for links that represent a physical
+	  relationship between two entities. The link may or may not be
+	  immutable, so applications must not assume either case.

include/uapi/linux/media.h

@@ -226,6 +226,7 @@ struct media_pad_desc {
 #define MEDIA_LNK_FL_LINK_TYPE			(0xf << 28)
 #  define MEDIA_LNK_FL_DATA_LINK		(0 << 28)
 #  define MEDIA_LNK_FL_INTERFACE_LINK		(1 << 28)
+#  define MEDIA_LNK_FL_ANCILLARY_LINK		(2 << 28)
 
 struct media_link_desc {
 	struct media_pad_desc source;