Re: [PATCH 4/5] [media] media-device: use kref for media_device instance

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

Em Thu, 17 Mar 2016 13:50:53 +0200
Sakari Ailus <sakari.ailus@iki.fi> escreveu:

> Hi Mauro,
> 
> On Wed, Mar 16, 2016 at 09:04:05AM -0300, Mauro Carvalho Chehab wrote:
> > Now that the media_device can be used by multiple drivers,
> > via devres, we need to be sure that it will be dropped only
> > when all drivers stop using it.  
> 
> Not long ago we split media device initialisation and registration into two.
> The intent with that was to prevent users from seeing the media device until
> all the initialisation is finished. I suppose that with dynamic updates, or
> media device being shared with two drivers, it might be difficult to achieve
> that. At least the method has to be different.
> 
> media_device_init() and media_device_cleanup() were a pair where the latter
> undid the first. 

Yes, this patch breaks the balance of _init()/_cleanup(), but this is for
a greater good. It is very hard to remove the devnode and cleanup the
media_device struct when multiple drivers can use it. The best way to
warrant that we'll be doing that is via kref.

> This patchs remove the requirement of calling cleanup
> explitly, breaking that model. It's perhaps not a big problem, there is
> likely no single driver which would initialise the media controller device
> once but would register and unregister it multiple times. I still wonder if
> we really lost something important if we removed media_device_init() and
> media_device_cleanup() altogether and merged the functionality in
> media_device_{,un}register().
> 
> Cc Javier who wrote the patch.

That patch was written to avoid a race trouble of having the media
devnode to initialize too early. Merging them back would cause troubles.

Maybe what we could do, instead, is to rename media_device_init()
to media_device_register(), and call the function that creates the
devnode as media_device_create_devnode().

What do you think?

> 
> commit 9832e155f1ed3030fdfaa19e72c06472dc2ecb1d
> Author: Javier Martinez Canillas <javier@osg.samsung.com>
> Date:   Fri Dec 11 20:57:08 2015 -0200
> 
>     [media] media-device: split media initialization and registration
> 
> For system-wide media device, I think we'd need to introduce a new concept,
> graph, that would define an interconnected set of entities. This would
> mainly be needed for callbacks, e.g. the power on / off sequences of the
> entities could be specific to V4L as discussed earlier. With this approach
> hacks wouldn't be needed to be made in the first place to support two drivers
> sharing a media device.
> 
> What was the original reason you needed to share the media device btw.?
> 
> > 
> > Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
> > ---
> >  drivers/media/media-device.c | 48 +++++++++++++++++++++++++++++++-------------
> >  include/media/media-device.h |  3 +++
> >  2 files changed, 37 insertions(+), 14 deletions(-)
> > 
> > diff --git a/drivers/media/media-device.c b/drivers/media/media-device.c
> > index c32fa15cc76e..38e6c319fe6e 100644
> > --- a/drivers/media/media-device.c
> > +++ b/drivers/media/media-device.c
> > @@ -721,6 +721,15 @@ int __must_check __media_device_register(struct media_device *mdev,
> >  {
> >  	int ret;
> >  
> > +	/* Check if mdev was ever registered at all */
> > +	mutex_lock(&mdev->graph_mutex);
> > +	if (media_devnode_is_registered(&mdev->devnode)) {
> > +		kref_get(&mdev->kref);
> > +		mutex_unlock(&mdev->graph_mutex);
> > +		return 0;
> > +	}
> > +	kref_init(&mdev->kref);
> > +
> >  	/* Register the device node. */
> >  	mdev->devnode.fops = &media_device_fops;
> >  	mdev->devnode.parent = mdev->dev;
> > @@ -730,8 +739,10 @@ int __must_check __media_device_register(struct media_device *mdev,
> >  	mdev->topology_version = 0;
> >  
> >  	ret = media_devnode_register(&mdev->devnode, owner);
> > -	if (ret < 0)
> > +	if (ret < 0) {
> > +		media_devnode_unregister(&mdev->devnode);
> >  		return ret;
> > +	}
> >  
> >  	ret = device_create_file(&mdev->devnode.dev, &dev_attr_model);
> >  	if (ret < 0) {
> > @@ -739,6 +750,7 @@ int __must_check __media_device_register(struct media_device *mdev,
> >  		return ret;
> >  	}
> >  
> > +	mutex_unlock(&mdev->graph_mutex);
> >  	dev_dbg(mdev->dev, "Media device registered\n");
> >  
> >  	return 0;
> > @@ -773,23 +785,15 @@ void media_device_unregister_entity_notify(struct media_device *mdev,
> >  }
> >  EXPORT_SYMBOL_GPL(media_device_unregister_entity_notify);
> >  
> > -void media_device_unregister(struct media_device *mdev)
> > +static void do_media_device_unregister(struct kref *kref)
> >  {
> > +	struct media_device *mdev;
> >  	struct media_entity *entity;
> >  	struct media_entity *next;
> >  	struct media_interface *intf, *tmp_intf;
> >  	struct media_entity_notify *notify, *nextp;
> >  
> > -	if (mdev == NULL)
> > -		return;
> > -
> > -	mutex_lock(&mdev->graph_mutex);
> > -
> > -	/* Check if mdev was ever registered at all */
> > -	if (!media_devnode_is_registered(&mdev->devnode)) {
> > -		mutex_unlock(&mdev->graph_mutex);
> > -		return;
> > -	}
> > +	mdev = container_of(kref, struct media_device, kref);
> >  
> >  	/* Remove all entities from the media device */
> >  	list_for_each_entry_safe(entity, next, &mdev->entities, graph_obj.list)
> > @@ -807,13 +811,26 @@ void media_device_unregister(struct media_device *mdev)
> >  		kfree(intf);
> >  	}
> >  
> > -	mutex_unlock(&mdev->graph_mutex);
> > +	/* Check if mdev devnode was registered */
> > +	if (!media_devnode_is_registered(&mdev->devnode))
> > +		return;
> >  
> >  	device_remove_file(&mdev->devnode.dev, &dev_attr_model);
> >  	media_devnode_unregister(&mdev->devnode);
> >  
> >  	dev_dbg(mdev->dev, "Media device unregistered\n");
> >  }
> > +
> > +void media_device_unregister(struct media_device *mdev)
> > +{
> > +	if (mdev == NULL)
> > +		return;
> > +
> > +	mutex_lock(&mdev->graph_mutex);
> > +	kref_put(&mdev->kref, do_media_device_unregister);
> > +	mutex_unlock(&mdev->graph_mutex);
> > +
> > +}
> >  EXPORT_SYMBOL_GPL(media_device_unregister);
> >  
> >  static void media_device_release_devres(struct device *dev, void *res)
> > @@ -825,13 +842,16 @@ struct media_device *media_device_get_devres(struct device *dev)
> >  	struct media_device *mdev;
> >  
> >  	mdev = devres_find(dev, media_device_release_devres, NULL, NULL);
> > -	if (mdev)
> > +	if (mdev) {
> > +		kref_get(&mdev->kref);
> >  		return mdev;
> > +	}
> >  
> >  	mdev = devres_alloc(media_device_release_devres,
> >  				sizeof(struct media_device), GFP_KERNEL);
> >  	if (!mdev)
> >  		return NULL;
> > +
> >  	return devres_get(dev, mdev, NULL, NULL);
> >  }
> >  EXPORT_SYMBOL_GPL(media_device_get_devres);
> > diff --git a/include/media/media-device.h b/include/media/media-device.h
> > index ca3871b853ba..73c16e6e6b6b 100644
> > --- a/include/media/media-device.h
> > +++ b/include/media/media-device.h
> > @@ -23,6 +23,7 @@
> >  #ifndef _MEDIA_DEVICE_H
> >  #define _MEDIA_DEVICE_H
> >  
> > +#include <linux/kref.h>
> >  #include <linux/list.h>
> >  #include <linux/mutex.h>
> >  
> > @@ -283,6 +284,7 @@ struct media_entity_notify {
> >   * struct media_device - Media device
> >   * @dev:	Parent device
> >   * @devnode:	Media device node
> > + * @kref:	Object refcount
> >   * @driver_name: Optional device driver name. If not set, calls to
> >   *		%MEDIA_IOC_DEVICE_INFO will return dev->driver->name.
> >   *		This is needed for USB drivers for example, as otherwise
> > @@ -347,6 +349,7 @@ struct media_device {
> >  	/* dev->driver_data points to this struct. */
> >  	struct device *dev;
> >  	struct media_devnode devnode;
> > +	struct kref kref;
> >  
> >  	char model[32];
> >  	char driver_name[32];
> > -- 
> > 2.5.0
> >   
> 


-- 
Thanks,
Mauro