Re: [media-workshop] [PATCH v8.4 81/83] [media] media-entity: init pads on entity init if was registered before

[Mauro Carvalho Chehab <mchehab@osg.samsung.com>]

Em Mon, 23 Nov 2015 17:21:22 -0300
Javier Martinez Canillas <javier@osg.samsung.com> escreveu:

> Hello Laurent,
> 
> On 11/23/2015 01:20 PM, Laurent Pinchart wrote:
> > Hi Javier,
> > 
> > (Replying to linux-media instead of media-workshop, I can't find this patch in 
> > my linux-media folder)
> >
> > Thank you for the patch.
> >
> 
> Thanks for your feedback.
>  
> > On Monday 12 October 2015 13:44:10 Mauro Carvalho Chehab wrote:
> >> From: Javier Martinez Canillas <javier@osg.samsung.com>
> >>
> >> If an entity is registered with a media device before is initialized
> >> with media_device_register_entity(), the number of pads won't be set
> >> so media_device_register_entity() won't create pad objects and add
> >> it to the media device pads list.
> >>
> >> Do this at entity initialization time if the entity was registered
> >> before so the graph is complete and correct regardless of the order
> >> in which the entities are initialized and registered.
> > 
> > We now have two places where the pads graph objects are initialized and that 
> > looks like a bug to me as media_gobj_init() allocates IDs and, even worse, 
> > adds the entity to the media device entities list.
> >

As Sakari commented on one of his reviews, media_gobj_init() should
actually be renamed to media_gobj_create(), as it is the function that
creates and registers a graph object on a given media device.

The hole idea is that any kind of graph object should be created at one
single point: media_gobj_init - to be renamed to media_gobj_create(), as
proposed by Sakari.

There's indeed bug on the existing subdev drivers: they want to create
pads and links without even knowing to what media_device those will be
associated! Every driver should be doing, instead:

1) create the media_device (or get a reference to an already created one);
2) create the entities associated with the mdev;
3) create the pads associated with the already created entity(ies);
4) register the entity;
5) create the links associated with two already created pads.

The above workflow is the correct one, as all graph objects should
be added to the mdev graph object lists that will be used by the
ioctl handler.

Also, please notice that, if all drivers follow the above procedure,
we could also merge entity create and register functions into one,
with is, IMHO, a good thing, as entity creation/registration will
be atomic.

However, several subdevs do, instead:

1) create entity(ies);
2) create pads;
3) get a reference for the media device;
4) register the entity;
5) create the links associated with two already created pads.

Changing such logic is not trivial, and require tests with devices
that we don't have.

So, the approach we took is to allow the above workflow to keep
working. That's what this patch does.

If you think it is worth to push for a change on those drivers, we
could eventually add a printk_once() to warn that the driver is
doing the wrong thing, and give some time for the drivers maintainers
to fix their drivers, and remove the backward compat code after done.

> Yes but the idea of this patch was in fact to make it less error prone and
> more robust since entities could either be initialized and later registered
> or first be registered and then later initialized.
>  
> > We need to standardize on where graph objects are initialized across the 
> > different kind of objects and document this clearly otherwise drivers will get 
> > it wrong.
> >
> 
> I'm OK with having more strict rules of the order in which objects should
> be initialized and registered and force all drivers to follow these docs.
> 
> >> Suggested-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
> 
> This patch was suggested by Mauro so I'll also let him to comment what he
> prefers or if he has another opinion on this.
> 
> >> Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
> >> ---
> >>  drivers/media/media-entity.c | 10 ++++++++++
> >>  1 file changed, 10 insertions(+)
> >>
> >> diff --git a/drivers/media/media-entity.c b/drivers/media/media-entity.c
> >> index f28265864b76..2c984fb7d497 100644
> >> --- a/drivers/media/media-entity.c
> >> +++ b/drivers/media/media-entity.c
> >> @@ -238,6 +238,7 @@ int
> >>  media_entity_init(struct media_entity *entity, u16 num_pads,
> >>  		  struct media_pad *pads)
> >>  {
> >> +	struct media_device *mdev = entity->graph_obj.mdev;
> >>  	unsigned int i;
> >>
> >>  	entity->group_id = 0;
> >> @@ -246,11 +247,20 @@ media_entity_init(struct media_entity *entity, u16
> >> num_pads, entity->num_pads = num_pads;
> >>  	entity->pads = pads;
> >>
> >> +	if (mdev)
> >> +		spin_lock(&mdev->lock);
> >> +
> >>  	for (i = 0; i < num_pads; i++) {
> >>  		pads[i].entity = entity;
> >>  		pads[i].index = i;
> >> +		if (mdev)
> >> +			media_gobj_init(mdev, MEDIA_GRAPH_PAD,
> >> +					&entity->pads[i].graph_obj);
> >>  	}
> >>
> >> +	if (mdev)
> >> +		spin_unlock(&mdev->lock);
> >> +
> >>  	return 0;
> >>  }
> >>  EXPORT_SYMBOL_GPL(media_entity_init);
> > 
> 
> Best regards,