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Subject: Re: [PATCH 2/3] media: add media_device_unregister_put() interface
To: Hans Verkuil <hverkuil@xs4all.nl>, mchehab@osg.samsung.com,
        laurent.pinchart@ideasonboard.com, sakari.ailus@iki.fi,
        hans.verkuil@cisco.com, chehabrafael@gmail.com, javier@osg.samsung.com,
        inki.dae@samsung.com, g.liakhovetski@gmx.de, jh1009.sung@samsung.com
References: <cover.1463158822.git.shuahkh@osg.samsung.com>
 <14efd8cc91d49e34936fd227d1208429d16e3fa0.1463158822.git.shuahkh@osg.samsung.com>
 <5742EBDA.1010902@xs4all.nl>
Cc: linux-media@vger.kernel.org, linux-kernel@vger.kernel.org,
        Shuah Khan <shuahkh@osg.samsung.com>
From: Shuah Khan <shuahkh@osg.samsung.com>
Message-ID: <57448BA7.3040107@osg.samsung.com>
Date: Tue, 24 May 2016 11:13:11 -0600
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On 05/23/2016 05:39 AM, Hans Verkuil wrote:
> On 05/13/2016 07:09 PM, Shuah Khan wrote:
>> Add media_device_unregister_put() interface to release reference to a media
>> device allocated using the Media Device Allocator API. The media device is
>> unregistered and freed when the last driver that holds the reference to the
>> media device releases the reference. The media device is unregistered and
>> freed in the kref put handler.
>>
>> Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
>> ---
>>  drivers/media/media-device.c | 11 +++++++++++
>>  include/media/media-device.h | 15 +++++++++++++++
>>  2 files changed, 26 insertions(+)
>>
>> diff --git a/drivers/media/media-device.c b/drivers/media/media-device.c
>> index 33a9952..b5c279a 100644
>> --- a/drivers/media/media-device.c
>> +++ b/drivers/media/media-device.c
>> @@ -36,6 +36,7 @@
>>  #include <media/media-device.h>
>>  #include <media/media-devnode.h>
>>  #include <media/media-entity.h>
>> +#include <media/media-dev-allocator.h>
>>  
>>  #ifdef CONFIG_MEDIA_CONTROLLER
>>  
>> @@ -818,6 +819,16 @@ void media_device_unregister(struct media_device *mdev)
>>  }
>>  EXPORT_SYMBOL_GPL(media_device_unregister);
>>  
>> +void media_device_unregister_put(struct media_device *mdev)
>> +{
>> +	if (mdev == NULL)
>> +		return;
>> +
>> +	dev_dbg(mdev->dev, "%s: mdev %p\n", __func__, mdev);
>> +	media_device_put(mdev);
>> +}
>> +EXPORT_SYMBOL_GPL(media_device_unregister_put);
>> +
> 
> I don't really see the need for a new unregister_put function. The only thing
> it adds compared to media_device_put is the 'if (mdev == NULL)' check.
> 
> Is that check needed at all?

Yeah. My reasoning for keeping this for symmetry in drivers
with _register() and _unregister(). However, drivers already
required to call media_device_put() in _register error legs.

> 
> I would probably go for an API like this:
> 
> <brainstorm mode on>
> 
> /* Not sure if there should be a void *priv as the last argument */
> int (*mdev_init_fnc)(struct media_device *mdev, struct device *dev);
> 
> /* Perhaps a void *priv is needed to pass to mdev_init_fnc?
>    The callback is there to initialize the media_device and it's called
>    with the lock held.
> */
> struct media_device *media_device_allocate(struct device *dev, mdev_init_fnc fnc);
> 
> /* Helper function for usb devices */
> struct media_device *media_device_usb_allocate(struct usb_device *udev,
> 					       char *driver_name);
> 
> /* counterpart of media_device_allocate, that makes more sense than allocate/put IMHO */
> void media_device_release(struct media_device *mdev);

We already have a media_device_release() that gets used as devnode->release
back. I will use a different name, media_device_usb_free() or something
similar.

> 
> <brainstorm mode off>
> 
> Regards,
> 
> 	Hans
> 
>>  static void media_device_release_devres(struct device *dev, void *res)
>>  {
>>  }
>> diff --git a/include/media/media-device.h b/include/media/media-device.h
>> index f743ae2..8bd836e 100644
>> --- a/include/media/media-device.h
>> +++ b/include/media/media-device.h
>> @@ -499,6 +499,18 @@ int __must_check __media_device_register(struct media_device *mdev,
>>  void media_device_unregister(struct media_device *mdev);
>>  
>>  /**
>> + * media_device_unregister_put() - Unregisters a media device element
>> + *
>> + * @mdev:	pointer to struct &media_device
>> + *
>> + * Should be called to unregister media device allocated with Media Device
>> + * Allocator API media_device_get() interface.
>> + * It is safe to call this function on an unregistered (but initialised)
>> + * media device.
>> + */
>> +void media_device_unregister_put(struct media_device *mdev);
>> +
>> +/**
>>   * media_device_register_entity() - registers a media entity inside a
>>   *	previously registered media device.
>>   *
>> @@ -658,6 +670,9 @@ static inline int media_device_register(struct media_device *mdev)
>>  static inline void media_device_unregister(struct media_device *mdev)
>>  {
>>  }
>> +static inline void media_device_unregister_put(struct media_device *mdev)
>> +{
>> +}
>>  static inline int media_device_register_entity(struct media_device *mdev,
>>  						struct media_entity *entity)
>>  {
>>
> --
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