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

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Message-Id: <201011221719.38369.laurent.pinchart@ideasonboard.com>
Date:	Mon, 22 Nov 2010 17:19:37 +0100
From:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:	Andy Walls <awalls@...metrocast.net>
Cc:	Hans Verkuil <hverkuil@...all.nl>,
	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")

Hi Andy,

On Monday 22 November 2010 13:51:37 Andy Walls wrote:
> On Mon, 2010-11-22 at 12:36 +0100, Hans Verkuil wrote:
> > 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,
> > > > 
> > > > 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.
> 
> +1 POLLHUP
> 
> http://www.opengroup.org/onlinepubs/009695399/functions/poll.html
> 
> "POLLHUP
>         The device has been disconnected. [...]"
> 
> 
> My $0.02 below:
> The communication link with the device is closed due to circumstances
> that the OS considers normal operation.  The OS has, at some level, shut
> down its side of the communication link gracefully as well.
> 
> Just because the application or the OS cannot always predict when a USB
> disconnect may happen, doesn't mean it is an error.

POLLHUP seems to be the best option standard-wise, but it could be a problem 
for output devices. Applications use the write file descriptors set with 
select() to wait for a buffer to be ready. Unlike POLLERR, POLLHUP only 
reports an event on the read file descriptors set.

-- 
Regards,

Laurent Pinchart
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