Re: [PATCH] v4l2-async: add subnotifier registration for subdevices

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Hi Niklas,

Thank you for the patch.

Do you happen to have a driver that would use this, to see some example of
how the code is to be used?

Could you update the documentation in
Documentation/media/kapi/v4l2-subdev.rst, too?

On Fri, Apr 28, 2017 at 12:30:35AM +0200, Niklas Söderlund wrote:
> When registered() of v4l2_subdev_internal_ops is called the subdevice
> have access to the master devices v4l2_dev and it's called with the
> async frameworks list_lock held. In this context the subdevice can
> register its own notifiers to allow for incremental discovery of
> subdevices.
> 
> The master device registers the subdevices closest to itself in its
> notifier while the subdevice(s) themself register notifiers for there
> closest neighboring devices when they are registered. Using this
> incremental approach two problems can be solved.
> 
> 1. The master device no longer have to care how many subdevices exist in

s/subdevices/devices/ ?

A single sub-device driver can expose multiple sub-devices for a single
device.

>    the pipeline. It only needs to care about its closest subdevice and
>    arbitrary long pipelines can be created without having to adapt the
>    master device for each case.
> 
> 2. Subdevices which are represented as a single DT node but register
>    more then one subdevice can use this to further the pipeline
>    discovery. Since the subdevice driver is the only one who knows which
>    of its subdevices is linked with which subdevice of a neighboring DT
>    node.
> 
> To enable subdevices to register/unregister notifiers from the
> registered()/unregistered() callback v4l2_async_subnotifier_register()
> and v4l2_async_subnotifier_unregister() are added. These new notifier
> register functions are similar to the master device equivalent functions
> but run without taking the v4l2-async list_lock which already are held
> when he registered()/unregistered() callbacks are called.
> 
> Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@xxxxxxxxxxxx>
> ---
>  drivers/media/v4l2-core/v4l2-async.c | 91 +++++++++++++++++++++++++++++-------
>  include/media/v4l2-async.h           | 22 +++++++++
>  2 files changed, 95 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-async.c b/drivers/media/v4l2-core/v4l2-async.c
> index 96cc733f35ef72b0..d4a676a2935eb058 100644
> --- a/drivers/media/v4l2-core/v4l2-async.c
> +++ b/drivers/media/v4l2-core/v4l2-async.c
> @@ -136,12 +136,13 @@ static void v4l2_async_cleanup(struct v4l2_subdev *sd)
>  	sd->dev = NULL;
>  }
>  
> -int v4l2_async_notifier_register(struct v4l2_device *v4l2_dev,
> -				 struct v4l2_async_notifier *notifier)
> +static int v4l2_async_do_notifier_register(struct v4l2_device *v4l2_dev,
> +					   struct v4l2_async_notifier *notifier,
> +					   bool subnotifier)
>  {
>  	struct v4l2_subdev *sd, *tmp;
>  	struct v4l2_async_subdev *asd;
> -	int i;
> +	int found, i;

If you need a boolean value, you could use bool type.

>  
>  	if (!notifier->num_subdevs || notifier->num_subdevs > V4L2_MAX_SUBDEVS)
>  		return -EINVAL;
> @@ -168,32 +169,69 @@ int v4l2_async_notifier_register(struct v4l2_device *v4l2_dev,
>  		list_add_tail(&asd->list, &notifier->waiting);
>  	}
>  
> -	mutex_lock(&list_lock);
> +	if (!subnotifier)
> +		mutex_lock(&list_lock);

Just to be sure, I'd verify the mutex is indeed acquired.
lockdep_assert_held(mutex) ?

> +
> +	/*
> +	 * This function can be called recursively so the list
> +	 * might be modified in a recursive call. Start from the
> +	 * top of the list each iteration.
> +	 */
> +	found = 1;
> +	while (found) {
> +		found = 0;
>  
> -	list_for_each_entry_safe(sd, tmp, &subdev_list, async_list) {
> -		int ret;
> +		list_for_each_entry_safe(sd, tmp, &subdev_list, async_list) {
> +			int ret;
>  
> -		asd = v4l2_async_belongs(notifier, sd);
> -		if (!asd)
> -			continue;
> +			asd = v4l2_async_belongs(notifier, sd);
> +			if (!asd)
> +				continue;
>  
> -		ret = v4l2_async_test_notify(notifier, sd, asd);
> -		if (ret < 0) {
> -			mutex_unlock(&list_lock);
> -			return ret;
> +			ret = v4l2_async_test_notify(notifier, sd, asd);
> +			if (ret < 0) {
> +				if (!subnotifier)
> +					mutex_unlock(&list_lock);
> +				return ret;
> +			}
> +
> +			found = 1;
> +			break;
>  		}
>  	}
>  
>  	/* Keep also completed notifiers on the list */
>  	list_add(&notifier->list, &notifier_list);
>  
> -	mutex_unlock(&list_lock);
> +	if (!subnotifier)
> +		mutex_unlock(&list_lock);
>  
>  	return 0;
>  }
> +
> +int v4l2_async_subnotifier_register(struct v4l2_subdev *sd,
> +				    struct v4l2_async_notifier *notifier)
> +{
> +	if (!sd->v4l2_dev) {
> +		dev_err(sd->dev ? sd->dev : NULL,
> +			"Can't register subnotifier for without v4l2_dev\n");
> +		return -EINVAL;

When did this start happening? :-)

> +	}
> +
> +	return v4l2_async_do_notifier_register(sd->v4l2_dev, notifier, true);
> +}
> +EXPORT_SYMBOL(v4l2_async_subnotifier_register);
> +
> +int v4l2_async_notifier_register(struct v4l2_device *v4l2_dev,
> +				 struct v4l2_async_notifier *notifier)
> +{
> +	return v4l2_async_do_notifier_register(v4l2_dev, notifier, false);
> +}
>  EXPORT_SYMBOL(v4l2_async_notifier_register);
>  
> -void v4l2_async_notifier_unregister(struct v4l2_async_notifier *notifier)
> +static void
> +v4l2_async_do_notifier_unregister(struct v4l2_async_notifier *notifier,
> +				  bool subnotifier)
>  {
>  	struct v4l2_subdev *sd, *tmp;
>  	unsigned int notif_n_subdev = notifier->num_subdevs;
> @@ -210,7 +248,8 @@ void v4l2_async_notifier_unregister(struct v4l2_async_notifier *notifier)
>  			"Failed to allocate device cache!\n");
>  	}
>  
> -	mutex_lock(&list_lock);
> +	if (!subnotifier)
> +		mutex_lock(&list_lock);
>  
>  	list_del(&notifier->list);
>  
> @@ -237,15 +276,20 @@ void v4l2_async_notifier_unregister(struct v4l2_async_notifier *notifier)
>  			put_device(d);
>  	}
>  
> -	mutex_unlock(&list_lock);
> +	if (!subnotifier)
> +		mutex_unlock(&list_lock);
>  
>  	/*
>  	 * Call device_attach() to reprobe devices
>  	 *
>  	 * NOTE: If dev allocation fails, i is 0, and the whole loop won't be
>  	 * executed.
> +	 * TODO: If we are unregistering a subdevice notifier we can't reprobe
> +	 * since the lock_list is held by the master device and attaching that
> +	 * device would call v4l2_async_register_subdev() and end in a deadlock
> +	 * on list_lock.
>  	 */
> -	while (i--) {
> +	while (i-- && !subnotifier) {

Why is this not done for sub-notifiers?

That said, the code here looks really dubious. But that's out of scope of
the patchset.

>  		struct device *d = dev[i];
>  
>  		if (d && device_attach(d) < 0) {
> @@ -269,6 +313,17 @@ void v4l2_async_notifier_unregister(struct v4l2_async_notifier *notifier)
>  	 * upon notifier registration.
>  	 */
>  }
> +
> +void v4l2_async_subnotifier_unregister(struct v4l2_async_notifier *notifier)
> +{
> +	v4l2_async_do_notifier_unregister(notifier, true);
> +}
> +EXPORT_SYMBOL(v4l2_async_subnotifier_unregister);
> +
> +void v4l2_async_notifier_unregister(struct v4l2_async_notifier *notifier)
> +{
> +	v4l2_async_do_notifier_unregister(notifier, false);
> +}
>  EXPORT_SYMBOL(v4l2_async_notifier_unregister);
>  
>  int v4l2_async_register_subdev(struct v4l2_subdev *sd)
> diff --git a/include/media/v4l2-async.h b/include/media/v4l2-async.h
> index 8e2a236a4d039df6..dee070be59f211bd 100644
> --- a/include/media/v4l2-async.h
> +++ b/include/media/v4l2-async.h
> @@ -105,6 +105,18 @@ struct v4l2_async_notifier {
>  };
>  
>  /**
> + * v4l2_async_notifier_register - registers a subdevice asynchronous subnotifier
> + *
> + * @sd: pointer to &struct v4l2_subdev
> + * @notifier: pointer to &struct v4l2_async_notifier
> + *
> + * This function assumes the async list_lock is already locked, allowing
> + * it to be used from struct v4l2_subdev_internal_ops registered() callback.
> + */
> +int v4l2_async_subnotifier_register(struct v4l2_subdev *sd,
> +				    struct v4l2_async_notifier *notifier);
> +
> +/**
>   * v4l2_async_notifier_register - registers a subdevice asynchronous notifier
>   *
>   * @v4l2_dev: pointer to &struct v4l2_device
> @@ -114,6 +126,16 @@ int v4l2_async_notifier_register(struct v4l2_device *v4l2_dev,
>  				 struct v4l2_async_notifier *notifier);
>  
>  /**
> + * v4l2_async_subnotifier_unregister - unregisters a asynchronous subnotifier
> + *
> + * @notifier: pointer to &struct v4l2_async_notifier
> + *
> + * This function assumes the async list_lock is already locked, allowing
> + * it to be used from struct v4l2_subdev_internal_ops unregistered() callback.
> + */
> +void v4l2_async_subnotifier_unregister(struct v4l2_async_notifier *notifier);
> +
> +/**
>   * v4l2_async_notifier_unregister - unregisters a subdevice asynchronous notifier
>   *
>   * @notifier: pointer to &struct v4l2_async_notifier

-- 
Kind regards,

Sakari Ailus
e-mail: sakari.ailus@xxxxxx	XMPP: sailus@xxxxxxxxxxxxxx



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