linux-kernel - Re: [PATCH v5 1/3] [media] media-device: check before unregister if mdev was registered

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Message-ID: <566B5DFB.3020100@osg.samsung.com>
Date:	Fri, 11 Dec 2015 16:36:27 -0700
From:	Shuah Khan <shuahkh@....samsung.com>
To:	Javier Martinez Canillas <javier@....samsung.com>,
	linux-kernel@...r.kernel.org
Cc:	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	Sakari Ailus <sakari.ailus@...ux.intel.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Hans Verkuil <hans.verkuil@...co.com>,
	linux-media@...r.kernel.org, Shuah Khan <shuahkh@....samsung.com>
Subject: Re: [PATCH v5 1/3] [media] media-device: check before unregister if
 mdev was registered

On 12/11/2015 03:57 PM, Javier Martinez Canillas wrote:
> Most media functions that unregister, check if the corresponding register
> function succeed before. So these functions can safely be called even if a
> registration was never made or the component as already been unregistered.
> 
> Add the same check to media_device_unregister() function for consistency.
> 
> This will also allow to split the media_device_register() function in an
> initialization and registration functions without the need to change the
> generic cleanup functions and error code paths for all the media drivers.
> 
> Suggested-by: Sakari Ailus <sakari.ailus@...ux.intel.com>
> Signed-off-by: Javier Martinez Canillas <javier@....samsung.com>
> Acked-by: Sakari Ailus <sakari.ailus@...ux.intel.com>
> 
> ---
> 
> Changes in v5: None
> Changes in v4: None
> Changes in v3: None
> Changes in v2:
> - Reword the documentation for media_device_unregister(). Suggested by Sakari.
> - Added Sakari's Acked-by tag for patch #1.
> 
>  drivers/media/media-device.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/media/media-device.c b/drivers/media/media-device.c
> index c12481c753a0..11c1c7383361 100644
> --- a/drivers/media/media-device.c
> +++ b/drivers/media/media-device.c
> @@ -577,6 +577,8 @@ EXPORT_SYMBOL_GPL(__media_device_register);
>   * media_device_unregister - unregister a media device
>   * @mdev:	The media device
>   *
> + * It is safe to call this function on an unregistered
> + * (but initialised) media device.
>   */
>  void media_device_unregister(struct media_device *mdev)
>  {
> @@ -584,6 +586,10 @@ void media_device_unregister(struct media_device *mdev)
>  	struct media_entity *next;
>  	struct media_interface *intf, *tmp_intf;
>  
> +	/* Check if mdev was ever registered at all */
> +	if (!media_devnode_is_registered(&mdev->devnode))
> +		return;

This is a good check, however, this check will not prevent
media_device_unregister() from getting run twice by two
different drivers. i.e media_devnode gets unregistered
towards the end of at the end of media_device_unregister().

In an example case, if bridge driver and snd-usb-aduio both
call media_device_unregister(), this check won't help prevent
media_device_unregister() being done only once. I think we
need a different state variable in struct media_device
to ensure unregister is done only once.

> +
>  	/* Remove all entities from the media device */
>  	list_for_each_entry_safe(entity, next, &mdev->entities, graph_obj.list)
>  		media_device_unregister_entity(entity);
> 


-- 
Shuah Khan
Sr. Linux Kernel Developer
Open Source Innovation Group
Samsung Research America (Silicon Valley)
shuahkh@....samsung.com | (970) 217-8978
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