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

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Date:	Thu, 06 Jan 2011 19:24:27 -0500
From:	Andy Walls <awalls@...metrocast.net>
To:	Hans Verkuil <hverkuil@...all.nl>, Greg KH <gregkh@...e.de>
Cc:	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
	alsa-devel@...a-project.org, broonie@...nsource.wolfsonmicro.com,
	clemens@...isch.de, sakari.ailus@...well.research.nokia.com
Subject: Re: [RFC/PATCH v7 01/12] media: Media device node support

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@...all.nl> 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
>--
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