Why, yes, there is a standard: http://pubs.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_03.html A somewhat verbose description of the errnos is in section 2.3. -Andy Hans Verkuil <hverkuil@xxxxxxxxx> 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? > >Regards, > > Hans > >-- >Hans Verkuil - video4linux developer - sponsored by Cisco >-- >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 ÿô.nlj·Ÿ®‰†+%ŠË±é¥Šwÿº{.nlj·¥Š{±þg‰¯â^n‡r¡öë¨è&£ûz¹Þúzf£¢·hšˆ§~††Ûÿÿïÿ‘ê_èæ+v‰¨þ)ßø
