[media] media: Add pad flag MEDIA_PAD_FL_MUST_CONNECT

Pads that set this flag must be connected by an active link for the entity
to stream.
Signed-off-by: NSakari Ailus <sakari.ailus@iki.fi>
Acked-by: NSylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>

Documentation/DocBook/media/v4l/media-ioc-enum-links.xml

@@ -134,6 +134,15 @@
 	    <entry>Output pad, relative to the entity. Output pads source data
 	    and are origins of links.</entry>
 	  </row>
+	  <row>
+	    <entry><constant>MEDIA_PAD_FL_MUST_CONNECT</constant></entry>
+	    <entry>If this flag is set and the pad is linked to any other
+	    pad, then at least one of those links must be enabled for the
+	    entity to be able to stream. There could be temporary reasons
+	    (e.g. device configuration dependent) for the pad to need
+	    enabled links even when this flag isn't set; the absence of the
+	    flag doesn't imply there is none.</entry>
+	  </row>
 	</tbody>
       </tgroup>
     </table>

include/uapi/linux/media.h

@@ -98,6 +98,7 @@ struct media_entity_desc {
 
 #define MEDIA_PAD_FL_SINK		(1 << 0)
 #define MEDIA_PAD_FL_SOURCE		(1 << 1)
+#define MEDIA_PAD_FL_MUST_CONNECT	(1 << 2)
 
 struct media_pad_desc {
 	__u32 entity;		/* entity ID */