media: mc: Expand MUST_CONNECT flag to always require an enabled link

BugLink: https://bugs.launchpad.net/bugs/2060531

[ Upstream commit b3decc5ce7d778224d266423b542326ad469cb5f ]

The MEDIA_PAD_FL_MUST_CONNECT flag indicates that the pad requires an
enabled link to stream, but only if it has any link at all. This makes
little sense, as if a pad is part of a pipeline, there are very few use
cases for an active link to be mandatory only if links exist at all. A
review of in-tree drivers confirms they all need an enabled link for
pads marked with the MEDIA_PAD_FL_MUST_CONNECT flag.

Expand the scope of the flag by rejecting pads that have no links at
all. This requires modifying the pipeline build code to add those pads
to the pipeline.

Cc: stable@vger.kernel.org # 6.1
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>

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

@@ -375,12 +375,11 @@ Types and flags used to represent the media graph elements
 	  are origins of links.
 
     *  -  ``MEDIA_PAD_FL_MUST_CONNECT``
-       -  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.
+       -  If this flag is set, then for this pad to be able to stream, it must
+	  be connected by at least one enabled link. 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.
 
 
 One and only one of ``MEDIA_PAD_FL_SINK`` and ``MEDIA_PAD_FL_SOURCE``