[PATCH 37/55] media: mc-entity: Add a new helper function to get a remote pad

[Daniel Scally]

Hello all

On 14/06/2022 20:11, Paul Elder wrote:
> From: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
>
> The media_entity_remote_pad() helper function returns the first remote
> pad it find connected to a given pad. Beside being possibly ill-named
> (as it operates on a pad, not an entity) and 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 at ideasonboard.com>
> ---
>  Documentation/driver-api/media/mc-core.rst |  4 +-
>  drivers/media/mc/mc-entity.c               | 38 ++++++++++++++++++
>  include/media/media-entity.h               | 45 ++++++++++++++++++++++
>  3 files changed, 85 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/driver-api/media/mc-core.rst b/Documentation/driver-api/media/mc-core.rst
> index 02481a2513b9..a2d1e32e3abb 100644
> --- a/Documentation/driver-api/media/mc-core.rst
> +++ b/Documentation/driver-api/media/mc-core.rst
> @@ -186,8 +186,8 @@ 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()` and
> -:c:func:`media_entity_remote_pad()`.
> +(:c:func:`media_entity_find_link()`, :c:func:`media_entity_remote_pad()` and
> +:c:func:`media_entity_remote_source_pad()`).
>  
>  Use count and power handling
>  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> diff --git a/drivers/media/mc/mc-entity.c b/drivers/media/mc/mc-entity.c
> index 11f5207f73aa..1febf5a86be6 100644
> --- a/drivers/media/mc/mc-entity.c
> +++ b/drivers/media/mc/mc-entity.c
> @@ -9,6 +9,7 @@
>   */
>  
>  #include <linux/bitmap.h>
> +#include <linux/list.h>
>  #include <linux/property.h>
>  #include <linux/slab.h>
>  #include <media/media-entity.h>
> @@ -920,6 +921,43 @@ struct media_pad *media_entity_remote_pad(const struct media_pad *pad)
>  }
>  EXPORT_SYMBOL_GPL(media_entity_remote_pad);
>  
> +struct media_pad *
> +media_entity_remote_pad_unique(const struct media_entity *entity,
> +			       unsigned int type)
> +{
> +	struct media_pad *pad = NULL;
> +	struct media_link *link;
> +
> +	list_for_each_entry(link, &entity->links, list) {
> +		struct media_pad *local_pad;
> +		struct media_pad *remote_pad;
> +
> +		if (!(link->flags & MEDIA_LNK_FL_ENABLED))
> +			continue;
Does this need another guard here to make sure the link isn't an
ancillary link? Only likely to happen at least in the immediate future
where the entity represents a camera sensor, so possibly not applicable
here - I couldn't find where the new function is used in the series to
check. I _think_ it would actually work ok regardless...media_gobj
type-punning makes my brain ache, but I think the local_pad->entity ==
entity comparison would actually compare the entity->name member of the
entity at the end of an ancillary link to the entity parameter, not find
a match and so continue the loop without failing, but that feels a bit
sub-optimal.



> +
> +		if (type == MEDIA_PAD_FL_SOURCE) {
> +			local_pad = link->sink;
> +			remote_pad = link->source;
> +		} else {
> +			local_pad = link->source;
> +			remote_pad = link->sink;
> +		}
> +
> +		if (local_pad->entity == entity) {
> +			if (pad)
> +				return ERR_PTR(-ENOTUNIQ);
> +
> +			pad = remote_pad;
> +		}
> +	}
> +
> +	if (!pad)
> +		return ERR_PTR(-ENOLINK);
> +
> +	return pad;
> +}
> +EXPORT_SYMBOL_GPL(media_entity_remote_pad_unique);
> +
>  static void media_interface_init(struct media_device *mdev,
>  				 struct media_interface *intf,
>  				 u32 gobj_type,
> diff --git a/include/media/media-entity.h b/include/media/media-entity.h
> index a9a1c0ec5d1c..33d5f52719a0 100644
> --- a/include/media/media-entity.h
> +++ b/include/media/media-entity.h
> @@ -859,6 +859,51 @@ struct media_link *media_entity_find_link(struct media_pad *source,
>   */
>  struct media_pad *media_entity_remote_pad(const struct media_pad *pad);
>  
> +/**
> + * media_entity_remote_pad_unique - Find a remote pad connected to an entity
> + * @entity: The entity
> + * @type: The type of pad to find (MEDIA_PAD_FL_SINK or MEDIA_PAD_FL_SOURCE)
> + *
> + * Search for and return a remote pad of @type connected to @entity through an
> + * enabled link. If multiple (or no) remote pads match these criteria, an error
> + * is returned.
> + *
> + * The uniqueness constraint makes this helper function suitable for entities
> + * that support a single active source or sink at a time.
> + *
> + * 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_entity_remote_pad_unique(const struct media_entity *entity,
> +			       unsigned int type);
> +
> +/**
> + * media_entity_remote_source_pad - Find a remote source pad connected to an entity
> + * @entity: The entity
> + *
> + * Search for and return a remote source pad connected to @entity through an
> + * enabled link. If multiple (or no) remote pads match these criteria, an error
> + * is returned.
> + *
> + * The uniqueness constraint makes this helper function suitable for entities
> + * that support a single active source at a time.
> + *
> + * 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
> + */
> +static inline struct media_pad *
> +media_entity_remote_source_pad(const struct media_entity *entity)
> +{
> +	return media_entity_remote_pad_unique(entity, MEDIA_PAD_FL_SOURCE);
> +}
> +
>  /**
>   * media_entity_is_streaming - Test if an entity is part of a streaming pipeline
>   * @entity: The entity