Re: [RFC/PATCH v7 01/12] media: Media device node support

[Greg KH <gregkh@suse.de>]

On Fri, Jan 07, 2011 at 12:27:11AM +0100, Hans Verkuil wrote:
> On Thursday, January 06, 2011 23:19:12 Greg KH wrote:
> 
> <snip>
> 
> > > > > +static ssize_t media_read(struct file *filp, char __user *buf,
> > > > > +		size_t sz, loff_t *off)
> > > > > +{
> > > > > +	struct media_devnode *mdev = media_devnode_data(filp);
> > > > > +
> > > > > +	if (!mdev->fops->read)
> > > > > +		return -EINVAL;
> > > > > +	if (!media_devnode_is_registered(mdev))
> > > > > +		return -EIO;
> > > > 
> > > > How could this happen?
> > > 
> > > This can happen when a USB device is disconnected for instance.
> > 
> > But what's to keep that from happening on the next line as well?
> 
> Nothing.
> 
> > That
> > doesn't seem like a check you can ever be sure about, so I wouldn't do
> > it at all.
> 
> Actually, there is a reason why this was done for v4l (and now the media
> API): typically, once a USB disconnect happens V4L drivers will call
> video_unregister_device() which calls device_unregister. Afterwards the
> device node should reject any new file operations except for release().
> 
> Obviously, this check can be done in the driver as well, but doing this
> check in the V4L core has the advantage of 1) consistent return codes and
> 2) drivers no longer have to check.
> 
> Of course, since the disconnect can happen at any time drivers still need
> to be able to handle errors from the USB subsystem due to disconnects, but
> that is something they always have to do.
> 
> > 
> > > > And are you sure -EIO is correct?
> > > 
> > > -ENXIO is probably better (I always confuse that with -ENODEV).
> 
> I wondered why V4L uses -EIO and I think it is related to the V4L2 specification
> of the read() function:
> 
> EIO
> I/O error. This indicates some hardware problem or a failure to communicate with
> a remote device (USB camera etc.).
> 
> Well, I guess a disconnect can be seen as a failure to communicate :-)
> 
> I think that ENODEV is much better. After all, there is no device
> anymore after a disconnect.
> 
> Is there some standard for this?

I've always thought that -ENODEV was good for when the device
disappeared.

thanks,

greg k-h