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

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Hans,

On Thu, Jul 07, 2022 at 08:52:16AM +0200, Hans Verkuil wrote:
> On 6/25/22 19:00, Laurent Pinchart wrote:
> > On Fri, Jun 17, 2022 at 01:48:05PM +0200, Hans Verkuil wrote:
> >> On 6/14/22 21:11, Paul Elder wrote:
> >>> From: Laurent Pinchart <laurent.pinchart@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.
> >>
> >> Question: of all the callers of this function, are there any that really
> >> need media_entity_remote_pad() instead of media_pad_remote_pad_unique()?
> >>
> >> Would it be possible to replace all callers of the old function with the
> >> new function? If that's the case, then the _unique suffix can be dropped,
> >> since that would effectively be the default. And if a function is needed
> >> to handle the case where there are multiple enabled links, then a new
> >> function should be created.
> > 
> > I don't think so. media_entity_remote_pad() operates on a pad, switching
> > to media_pad_remote_pad_unique() wouldn't work on subdevs that have
> > multiple sink or source pads with one active link each.
> 
> Do we have those today in the mainline kernel? Just checking...

Re-reading my reply, it seems I may have been mistaken. We may be able
to ditch media_pad_remote_pad_first() and replace it with
media_pad_remote_pad_unique() (at least in the majority of cases), but
we'll need to carefully review each user.

> >> Also, media_entity_remote_pad() should really be renamed to
> >> media_pad_remote_pad_first() or something like that, right? I'm not saying
> >> you should, but that's really what it does, as I understand it.
> > 
> > Yes, I think that would make sense, and it would freethe
> > media_entity_remote_pad() name, so the new function wouldn't need the
> > _unique suffix. I'll give it a try.
> > 
> >>> 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>
> >>> ---
> >>>  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;
> >>> +
> >>> +		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

-- 
Regards,

Laurent Pinchart