Re: [PATCH v5 1/2] media: Add obj_type field to struct media_entity

Sakari Ailus
2016-03-23T16:18:13Z

Hi Mauro,

On Wed, Mar 23, 2016 at 12:00:59PM -0300, Mauro Carvalho Chehab wrote:
> Em Wed, 23 Mar 2016 15:05:41 +0100
> Hans Verkuil <hverkuil@xs4all.nl> escreveu:
>
> > On 03/23/2016 11:35 AM, Mauro Carvalho Chehab wrote:
> > > Em Wed, 23 Mar 2016 10:45:55 +0200
> > > Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com> escreveu:
> > >
> > >> Code that processes media entities can require knowledge of the
> > >> structure type that embeds a particular media entity instance in order
> > >> to cast the entity to the proper object type. This needs is shown by the
> > >> presence of the is_media_entity_v4l2_io and is_media_entity_v4l2_subdev
> > >> functions.
> > >>
> > >> The implementation of those two functions relies on the entity function
> > >> field, which is both a wrong and an inefficient design, without even
> > >> mentioning the maintenance issue involved in updating the functions
> > >> every time a new entity function is added. Fix this by adding add an
> > >> obj_type field to the media entity structure to carry the information.
> > >>
> > >> Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
> > >> Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
> > >> Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> > >> ---
> > >> drivers/media/media-device.c | 2 +
> > >> drivers/media/v4l2-core/v4l2-dev.c | 1 +
> > >> drivers/media/v4l2-core/v4l2-subdev.c | 1 +
> > >> include/media/media-entity.h | 79 +++++++++++++++++++----------------
> > >> 4 files changed, 46 insertions(+), 37 deletions(-)
> > >>
> > >> diff --git a/drivers/media/media-device.c b/drivers/media/media-device.c
> > >> index 4a97d92a7e7d..88d8de3b7a4f 100644
> > >> --- a/drivers/media/media-device.c
> > >> +++ b/drivers/media/media-device.c
> > >> @@ -580,6 +580,8 @@ int __must_check media_device_register_entity(struct media_device *mdev,
> > >> "Entity type for entity %s was not initialized!\n",
> > >> entity->name);
> > >>
> > >> + WARN_ON(entity->obj_type == MEDIA_ENTITY_TYPE_INVALID);
> > >> +
> > >
> > > This is not ok. There are valid cases where the entity is not embedded
> > > on some other struct. That's the case of connectors/connections, for
> > > example: a connector/connection entity doesn't need anything else but
> > > struct media device.
> >
> > Once connectors are enabled, then we do need a MEDIA_ENTITY_TYPE_CONNECTOR or
> > MEDIA_ENTITY_TYPE_STANDALONE or something along those lines.
> >
> > > Also, this is V4L2 specific. Neither ALSA nor DVB need to use container_of().
> > > Actually, this won't even work there, as the entity is stored as a pointer,
> > > and not as an embedded data.
> >
> > Any other subsystem that *does* embed this can use obj_type. If it doesn't embed
> > it in anything, then MEDIA_ENTITY_TYPE_STANDALONE should be used (or whatever
> > name we give it). I agree that we need a type define for the case where it is
> > not embedded.
> >
> > >
> > > So, if we're willing to do this, then it should, instead, create
> > > something like:
> > >
> > > struct embedded_media_entity {
> > > struct media_entity entity;
> > > enum media_entity_type obj_type;
> > > };
> >
> > It's not v4l2 specific. It is just that v4l2 is the only subsystem that requires
> > this information at the moment. I see no reason at all to create such an ugly
> > struct.
>
> At the minute we added a BUG_ON() there, it became mandatory that all
> struct media_entity to be embedded. This is not always true, but
> as the intention is to avoid the risk of embedding it without a type,
> it makes sense to have the above struct. This way, the obj_type
> usage will be enforced *only* in the places where it is needed.
>
> We could, instead, remove BUG_ON() and make MEDIA_ENTITY_TYPE_STANDALONE
> the default type, but that won't enforce its usage where it is needed.

My understanding, based on the previous discussion, was that the
media_entity would in practice be always embedded in another struct. If
that's not the case, I think Hans proposed a few good options for type enums
telling the media_entity is not embedded in another struct.

For what it's worth, my personal favourite is "NONE".

>
> > I very strongly suspect that other subsystems will also embed this in their own
> > internal structs.
>
> They will if they need.
>
> > I actually wonder why it isn't embedded in struct dvb_device,
> > but I have to admit that I didn't take a close look at that. The pads are embedded
> > there, so it is somewhat odd that the entity isn't.
>
> The only advantage of embedding instead of using a pointer is that
> it would allow to use container_of() to get the struct. On the
> other hand, there's one drawback: both container and embedded
> structs will be destroyed at the same time. This can be a problem
> if the embedded object needs to live longer than the container.
>
> Also, the usage of container_of() doesn't work fine if the
> container have embedded two objects of the same type.
>
> In the specific case of DVB, let's imagine we would use the above
> solution and add a MEDIA_ENTITY_TYPE_DVB_DEVICE.
>
> If you look into struct dvb_device, you'll see that there are
> actually two media_entities on it:
>
> struct dvb_device {
> ...
> struct media_entity *entity, *tsout_entity;
> ...
> };
>
> If we had embedded them, just knowing that the container is
> struct dvb_device won't help, as the offsets for "entity"
> and for "tsout_entity" to get its container would be different.
>
> OK, we could have added two types there, but all of these
> would be just adding uneeded complexity and wound't be error
> prone. Also, there's no need to use container_of(), as a pointer
> to the dvb_device struct is always there at the DVB code.

If you wanted to obtain the dvb_device here, you'd instead add another
struct that embeds the struct media_entity. This struct would contain a
pointer to struct dvb_device. Simple and efficient.

>
> The same happens at ALSA code: so far, there's no need to go from a
> media_entity to its container.
>
> So, as I said before, the usage of container_of() and the need for an
> object type is currently V4L2 specific, and it is due to the way
> the v4l2 core and subdev framework was modeled. Don't expect or
> force that all subsystems would do the same.

What you do miss with using a pointer above is to find sub-system specific
information related to the entity. It's a very common operation, if there is
sub-system specific information related to an entity. I would say that if
you don't, the driver creating the media device must be aware of a lot of
the properties of the devices the media graph consists of, or use
alternative means to associate subsystem specific information to entities.
Looping over a linked list, for instance, is bound to be less efficient.

I don't see anything V4L2 specific here as such, even though no other
sub-system would (yet) need the feature.

--
Kind regards,

Sakari Ailus
e-mail: sakari.ailus@iki.fi XMPP: sailus@retiisi.org.uk