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 -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html