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

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

Hi Mauro,

On Wednesday 23 Mar 2016 12:00:59 Mauro Carvalho Chehab wrote:
> Em Wed, 23 Mar 2016 15:05:41 +0100 Hans Verkuil escreveu:
> > On 03/23/2016 11:35 AM, Mauro Carvalho Chehab wrote:
> >> Em Wed, 23 Mar 2016 10:45:55 +0200 Laurent Pinchart 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,

Note that it's a WARN_ON(), not a BUG_ON().

> it became mandatory that all struct media_entity to be embedded.

No, it becomes mandatory to initialize the field.

> 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.
> 
> > 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.

That's not the issue. The two entities above do not represent the DVB device, 
struct dvb_device should certainly not inherit from media_entity. Those two 
entities should be embedded in the DVB structures that model the objects they 
represented.

> 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.
> 
> 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.

It's not a V4L2-specific concept, it's an OOP concept. The fact that the very 
few users of media entities outside of V4L2 don't currently embed struct 
media_entity doesn't change anything here.

-- 
Regards,

Laurent Pinchart