[libcamera-devel] [PATCH 5/5] media: v4l2-async: Create links during v4l2_async_match_notify()

[Laurent Pinchart]

Hi Dan,

On Tue, Dec 14, 2021 at 11:41:27PM +0000, Daniel Scally wrote:
> On 14/12/2021 23:01, Laurent Pinchart wrote:
> > On Tue, Dec 14, 2021 at 10:36:01PM +0000, Daniel Scally wrote:
> >> On 14/12/2021 22:22, Laurent Pinchart wrote:
> >>> On Mon, Dec 13, 2021 at 11:28:49PM +0000, Daniel Scally wrote:
> >>>> Upon an async fwnode match, there's some typical behaviour that the
> >>>> notifier and matching subdev will want to do. For example, a notifier
> >>>> representing a sensor matching to an async subdev representing its
> >>>> VCM will want to create an ancillary link to expose that relationship
> >>>> to userspace.
> >>>>
> >>>> To avoid lots of code in individual drivers, try to build these links
> >>>> within v4l2 core.
> >>>>
> >>>> Signed-off-by: Daniel Scally <djrscally at gmail.com>
> >>>> ---
> >>>> Changes since the rfc:
> >>>>
> >>>> 	- None
> >>>>
> >>>>  drivers/media/v4l2-core/v4l2-async.c | 51 ++++++++++++++++++++++++++++
> >>>>  1 file changed, 51 insertions(+)
> >>>>
> >>>> diff --git a/drivers/media/v4l2-core/v4l2-async.c b/drivers/media/v4l2-core/v4l2-async.c
> >>>> index 0404267f1ae4..6575b1cbe95f 100644
> >>>> --- a/drivers/media/v4l2-core/v4l2-async.c
> >>>> +++ b/drivers/media/v4l2-core/v4l2-async.c
> >>>> @@ -275,6 +275,45 @@ v4l2_async_nf_try_complete(struct v4l2_async_notifier *notifier)
> >>>>  static int
> >>>>  v4l2_async_nf_try_all_subdevs(struct v4l2_async_notifier *notifier);
> >>>>  
> >>>> +static int
> >>>> +__v4l2_async_create_ancillary_link(struct v4l2_async_notifier *notifier,
> >>>> +				   struct v4l2_subdev *sd)
> >>>> +{
> >>>> +	struct media_link *link;
> >>>> +
> >>>> +	if (sd->entity.function != MEDIA_ENT_F_LENS &&
> >>>> +	    sd->entity.function != MEDIA_ENT_F_FLASH)
> >>>> +		return -EINVAL;
> >>>> +
> >>>> +	link = media_create_ancillary_link(&notifier->sd->entity, &sd->entity,
> >>>
> >>> Is there a guarantee at this point that notifier->sd->entity has already
> >>> been registered with the media_device ? That's done by
> >>> media_device_register_entity() called from
> >>> v4l2_device_register_subdev().
> >>
> >> v4l2_async_match_notify() calls v4l2_device_register_subdev() before the
> >> point that I've added the call to v4l2_async_try_create_links(), so I
> >> think that's covered there.
> >
> > It calls it on sd, not notifier->sd. It's the latter that concerns me.
> 
> Ah, you're right of course...I guess in that case the notifier->sd would
> get registered during the v4l2_async_match_notify() where the sensor
> driver's subdev is sd, but I don't think there's any guarantee that that
> would happen first...I haven't traced it through but my guess is that it
> would depend on the order in which the ipu3-cio2, sensor and lens
> controller drivers probed. I'll check to try and be sure how it works
> tomorrow

I was looking at media_device_register_entity(), and it sets

	INIT_LIST_HEAD(&entity->links);
	entity->num_links = 0;
	entity->num_backlinks = 0;

If we create links before that, things may go bad.

> >>>> +					   MEDIA_LNK_FL_ENABLED |
> >>>> +					   MEDIA_LNK_FL_IMMUTABLE);
> >>>> +
> >>>> +	return IS_ERR(link) ? PTR_ERR(link) : 0;
> >>>> +}
> >>>> +
> >>>> +/*
> >>>> + * Setup links on behalf of the notifier and subdev, where it's obvious what
> >>>
> >>> s/Setup/Create/ ("link setup" refers to another operation, enabling and
> >>> disabling links at runtime)
> >>
> >> Yes, good point; although that too is a piece of terminology I find a
> >> bit jarring to be honest; I would have named that function
> >> media_entity_configure_link()
> >>
> >>>> + * should be done. At the moment, we only support cases where the notifier
> >>>> + * is a sensor and the subdev is a lens.
> >>>
> >>> s/sensor/camera sensor/
> >>> s/lens/lens controller/
> >>>
> >>>> + *
> >>>> + * TODO: Setup pad links if the notifier's function is MEDIA_ENT_F_VID_IF_BRIDGE
> >>>> + * and the subdev's is MEDIA_ENT_F_CAM_SENSOR
> >>>> + */
> >>>> +static int v4l2_async_try_create_links(struct v4l2_async_notifier *notifier,
> >>>> +				       struct v4l2_subdev *sd)
> >>>> +{
> >>>> +	if (!notifier->sd)
> >>>> +		return 0;
> >>>> +
> >>>> +	switch (notifier->sd->entity.function) {
> >>>> +	case MEDIA_ENT_F_CAM_SENSOR:
> >>>> +		return __v4l2_async_create_ancillary_link(notifier, sd);
> >>>> +	default:
> >>>> +		return 0;
> >>>> +	}
> >>>> +}
> >>>> +
> >>>>  static int v4l2_async_match_notify(struct v4l2_async_notifier *notifier,
> >>>>  				   struct v4l2_device *v4l2_dev,
> >>>>  				   struct v4l2_subdev *sd,
> >>>> @@ -293,6 +332,18 @@ static int v4l2_async_match_notify(struct v4l2_async_notifier *notifier,
> >>>>  		return ret;
> >>>>  	}
> >>>>  
> >>>> +	/*
> >>>> +	 * Depending of the function of the entities involved, we may want to
> >>>> +	 * create links between them (for example between a sensor and its lens
> >>>> +	 * or between a sensor's source pad and the connected device's sink
> >>>> +	 * pad)
> >>>
> >>> s/)/)./
> >>>
> >>>> +	 */
> >>>> +	ret = v4l2_async_try_create_links(notifier, sd);
> >>>> +	if (ret) {
> >>>> +		v4l2_device_unregister_subdev(sd);
> >>>> +		return ret;
> >>>> +	}
> >>>> +
> >>>>  	/* Remove from the waiting list */
> >>>>  	list_del(&asd->list);
> >>>>  	sd->asd = asd;

-- 
Regards,

Laurent Pinchart