[PATCH 2/2] [media] media-device: split media initialization and registration

[Sakari Ailus]

Hi Mauro,

On Tue, Dec 15, 2015 at 09:13:42AM -0200, Mauro Carvalho Chehab wrote:
> Em Thu, 10 Sep 2015 20:14:04 +0300
> Sakari Ailus <sakari.ailus at linux.intel.com> escreveu:
> 
> > Hi Javier,
> > 
> > Thanks for the set! A few comments below.
> > 
> > Javier Martinez Canillas wrote:
> > > The media device node is registered and so made visible to user-space
> > > before entities are registered and links created which means that the
> > > media graph obtained by user-space could be only partially enumerated
> > > if that happens too early before all the graph has been created.
> > > 
> > > To avoid this race condition, split the media init and registration
> > > in separate functions and only register the media device node when
> > > all the pending subdevices have been registered, either explicitly
> > > by the driver or asynchronously using v4l2_async_register_subdev().
> > > 
> > > Also, add a media_entity_cleanup() function that will destroy the
> > > graph_mutex that is initialized in media_entity_init().
> > > 
> > > Suggested-by: Sakari Ailus <sakari.ailus at linux.intel.com>
> > > Signed-off-by: Javier Martinez Canillas <javier at osg.samsung.com>
> > > 
> > > ---
> > > 
> > >  drivers/media/common/siano/smsdvb-main.c      |  1 +
> > >  drivers/media/media-device.c                  | 38 +++++++++++++++++++++++----
> > >  drivers/media/platform/exynos4-is/media-dev.c | 12 ++++++---
> > >  drivers/media/platform/omap3isp/isp.c         | 11 +++++---
> > >  drivers/media/platform/s3c-camif/camif-core.c | 13 ++++++---
> > >  drivers/media/platform/vsp1/vsp1_drv.c        | 19 ++++++++++----
> > >  drivers/media/platform/xilinx/xilinx-vipp.c   | 11 +++++---
> > >  drivers/media/usb/au0828/au0828-core.c        | 26 +++++++++++++-----
> > >  drivers/media/usb/cx231xx/cx231xx-cards.c     | 22 +++++++++++-----
> > >  drivers/media/usb/dvb-usb-v2/dvb_usb_core.c   | 11 +++++---
> > >  drivers/media/usb/dvb-usb/dvb-usb-dvb.c       | 13 ++++++---
> > >  drivers/media/usb/siano/smsusb.c              | 14 ++++++++--
> > >  drivers/media/usb/uvc/uvc_driver.c            |  9 +++++--
> > >  include/media/media-device.h                  |  2 ++
> > >  14 files changed, 156 insertions(+), 46 deletions(-)
> > > 
> > > diff --git a/drivers/media/common/siano/smsdvb-main.c b/drivers/media/common/siano/smsdvb-main.c
> > > index ab345490a43a..8a1ea2192439 100644
> > > --- a/drivers/media/common/siano/smsdvb-main.c
> > > +++ b/drivers/media/common/siano/smsdvb-main.c
> > > @@ -617,6 +617,7 @@ static void smsdvb_media_device_unregister(struct smsdvb_client_t *client)
> > >  	if (!coredev->media_dev)
> > >  		return;
> > >  	media_device_unregister(coredev->media_dev);
> > > +	media_device_cleanup(coredev->media_dev);
> > >  	kfree(coredev->media_dev);
> > >  	coredev->media_dev = NULL;
> > >  #endif
> > > diff --git a/drivers/media/media-device.c b/drivers/media/media-device.c
> > > index 745defb34b33..a8beb0b445a6 100644
> > > --- a/drivers/media/media-device.c
> > > +++ b/drivers/media/media-device.c
> > > @@ -526,7 +526,7 @@ static void media_device_release(struct media_devnode *mdev)
> > >  }
> > >  
> > >  /**
> > > - * media_device_register - register a media device
> > > + * media_device_init() - initialize a media device
> > >   * @mdev:	The media device
> > >   *
> > >   * The caller is responsible for initializing the media device before
> > > @@ -534,12 +534,11 @@ static void media_device_release(struct media_devnode *mdev)
> > >   *
> > >   * - dev must point to the parent device
> > >   * - model must be filled with the device model name
> > > + *
> > > + * returns zero on success or a negative error code.
> > >   */
> > > -int __must_check __media_device_register(struct media_device *mdev,
> > > -					 struct module *owner)
> > > +int __must_check media_device_init(struct media_device *mdev)
> > 
> > I think I suggested making media_device_init() return void as the only
> > remaining source of errors would be driver bugs.
> > 
> > I'd simply replace the WARN_ON() below with BUG().
> 
> That sounds like bad idea to me, and it is against the current
> Kernel policy of using BUG() only when there's no other way, e. g. on
> event so severe that the Kernel has no other thing to do except to
> stop running.
> 
> For sure, this is not the case here. Also, all drivers have already
> a logic that checks if the device init happened. So, they should already
> be doing the right thing.

My point is that it's simply counter-productive to require the caller to
perform error handling in cases such as the only possible source of the
error being a NULL argument passed to the callee.

To give you some examples, device_register(), device_add() nor mutex_lock()
perform such checks. Some functions in V4L2 do, but I understand that's
sometimes for historical reasons where NULL arguments were allowed. Or that
there are other possible sources for errors in non-trivial functions and the
rest of the checks are done on the side.

If you don't like BUG_ON(), just drop it. It's as simple as that.

If there are other sources of errors then the matter is naturally entirely
different.

-- 
Kind regards,

Sakari Ailus
e-mail: sakari.ailus at iki.fi	XMPP: sailus at retiisi.org.uk