linux-kernel - Re: What should poll() return when a device is unregistered ? (was "media: Media device node support")

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Message-Id: <201011221236.40194.hverkuil@xs4all.nl>
Date:	Mon, 22 Nov 2010 12:36:39 +0100
From:	Hans Verkuil <hverkuil@...all.nl>
To:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:	Jonathan Corbet <corbet@....net>, linux-media@...r.kernel.org,
	sakari.ailus@...well.research.nokia.com,
	linux-kernel@...r.kernel.org
Subject: Re: What should poll() return when a device is unregistered ? (was "media: Media device node support")

On Monday, November 22, 2010 11:41:27 Laurent Pinchart wrote:
> Hi Hans,
> 
> On Monday 22 November 2010 10:08:06 Hans Verkuil wrote:
> > On Monday, November 22, 2010 00:35:54 Laurent Pinchart wrote:
> > > Hi Jonathan,
> > > 
> > > I forgot to answer one of your comments.
> > > 
> > > On Wednesday 17 November 2010 01:31:15 Jonathan Corbet wrote:
> > > 
> > > [snip]
> > > 
> > > > > +static unsigned int media_poll(struct file *filp,
> > > > > +			       struct poll_table_struct *poll)
> > > > > +{
> > > > > +	struct media_devnode *mdev = media_devnode_data(filp);
> > > > > +
> > > > > +	if (!mdev->fops->poll || !media_devnode_is_registered(mdev))
> > > > > +		return DEFAULT_POLLMASK;
> > > > > +	return mdev->fops->poll(filp, poll);
> > > > > +}
> > > > 
> > > > If it's not registered, I would expect poll() to return an error.
> > > 
> > > Agreed. I'll return POLLERR | POLLHUP in that case. Is that fine with you
> > > ?
> > 
> > When I looked at this for the core code I decided to just return POLLERR.
> 
> I've copied the usbdevfs code which returns POLLERR | POLLHUP when devices are 
> disconnected.
> 
> > That seemed to be what the majority of other usb drivers do. ALSA returns
> > POLLERR | POLLNVAL, by the way, which I think is a poor choice.
> 
> Indeed, POLLNVAL has a clear semantics that doesn't apply here.

In fact, POLLNVAL is currently only set by ./sound/core/init.c and by
./drivers/media/video/zoran/zoran_driver.c. In both cases it's rather dubious.

> > This doesn't really seem to be standardized :-(
> 
> CC'ing LKML with the question.
> 
> POLLERR | POLLHUP and POLLERR won't make a difference to select(), but we 
> should still standardize on a poll() return code when devices are unregistered 
> and/or - for hot-pluggable devices - disconnected (for V4L devices 
> unregistered usually means disconnected) ?

Drivers return POLLERR, POLLERR|POLLHUP or POLLHUP in case of a disconnect.
I'm leaning towards POLLHUP as the most appropriate poll return value for a
USB disconnect.

Regards,

	Hans

-- 
Hans Verkuil - video4linux developer - sponsored by Cisco
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