media: mc: entity: Fix minor issues in comments and documentation

Commit ae219872834a ("media: mc: entity: Rewrite
media_pipeline_start()") incorrectly referred to entity instead of pad
in a comment, and forgot to update a second comment accordingly when
moving the pipe from entity to pad. Furthermore, it didn't properly
reflow the documentation text it updated.

Fix those small issues.

Fixes: ae219872834a ("media: mc: entity: Rewrite media_pipeline_start()")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

media: mc: entity: Rewrite media_pipeline_start()

[Note: the code is mostly from Laurent but the patch description is from Tomi]

The media_pipeline_start() and media_pipeline_stop() functions use the
media graph walk API to traverse the graph and validate the pipeline.
The graph walk traverses the media graph following links between the
entities.

Also, while the pipeline can't change between the start and stop calls,
the graph is walked again from scratch at stop time, or any time a
driver needs to inspect the pipeline.

With the upcoming multiplexed streams support we will need a bit more
intelligent pipeline construction, as e.g. two independent streams may
be passing through a single entity via separate pads in which case
those pads should not be part of the same pipeline.

This patch essentially rewrites the media_pipeline_start/stop so that
a pipeline is defined as a set of pads instead of entities and the media
graph traversal considers the pad interdependencies when choosing which
links to follow.

Currently all the entity's pads are considered as interdependent. This
means that the behavior with all the current drivers stays the same, but
in the future we can define a more fine-grained pipeline construction.

Additionally the media pipeline's pads are cached at
media_pipeline_start() time, and re-used at media_pipeline_stop() which
avoid the need to re-walk the whole graph as the previous implementation
did.

Also, caching pads in the pipeline can serve in the future as the
foundation to provide a better API than the media graph walk to drivers
to iterate over pads and entities in the pipeline.

Note that the old media_pipeline_start/stop used the media graph walk
API. The new version does not use the media graph walk API, but instead
a new implementation.

There are two reasons for not changing the graph walk: it proved to be
rather difficult to change the graph walk to have the features
implemented in this patch, and second, this keeps the backward
compatibility of the graph walk as there are users of the graph walk API

The long term plan is that all the existing code would be converted to
use the new cached pipeline, thus allowing us to remove the graph walk.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

media: Documentation: mc: add definitions for stream and pipeline

The doc talks about streams and pipelines, but doesn't really define
them. This is an attempt to define them according to my understanding.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

media: Documentation: mc-core: Fix typo

Fix a typo in the mc-core.rst media driver API documentation. Due to its
nature, the typo unfortunately caused a warning during documentation
build.

Fixes: 03b282861ca7 ("media: mc-entity: Add a new helper function to get a remote pad for a pad")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

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>

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

The media_entity_remote_pad_first() helper function returns the first
remote pad it finds connected to a given pad. Beside being possibly
non-deterministic (as it stops at the first enabled link), the fact that
it returns the first match makes it unsuitable for drivers that need to
guarantee that a single link is enabled, for instance when an entity can
process data from one of multiple sources at a time.

For those use cases, add a new helper function,
media_entity_remote_pad_unique(), that operates on an entity and returns
a remote pad, with a guarantee that only one link is enabled. To ease
its use in drivers, also add an inline wrapper that locates source pads
specifically. A wrapper that locates sink pads can easily be added when
needed.

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>

media: mc-entity: Rename media_entity_remote_pad() to media_pad_remote_pad_first()

The media_entity_remote_pad() is misnamed, as it operates on a pad and
not an entity. Rename it to media_pad_remote_pad_first() to clarify its
behaviour.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-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>

media: Documentation: mc: Add media_device_{init,cleanup}

Document that drivers must first initialise a media device before
registering it, and clean up once the media device is unregistered.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

docs: get rid of :c:type explicit declarations for structs

The :c:type:`foo` only works properly with structs before
Sphinx 3.x.

On Sphinx 3.x, structs should now be declared using the
.. c:struct, and referenced via :c:struct tag.

As we now have the automarkup.py macro, that automatically
convert:
struct foo

into cross-references, let's get rid of that, solving
several warnings when building docs with Sphinx 3.x.

Reviewed-by: André Almeida <andrealmeid@collabora.com> # blk-mq.rst
Reviewed-by: Takashi Iwai <tiwai@suse.de> # sound
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

media: docs: kAPI docs: move them to driver-api

All those documents describe the media driver API. So, move
them to the right place within the Kernel documentation.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>