linux-kernel - Re: [RFC PATCH] poll: add poll_requested

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Date:	Thu, 7 Jul 2011 18:46:33 +0200
From:	Hans Verkuil <hverkuil@...all.nl>
To:	Jonathan Corbet <corbet@....net>
Cc:	"linux-kernel" <linux-kernel@...r.kernel.org>,
	"linux-media" <linux-media@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk
Subject: Re: [RFC PATCH] poll: add poll_requested_events() function

On Thursday, July 07, 2011 18:42:55 Jonathan Corbet wrote:
> On Fri, 1 Jul 2011 15:37:30 +0200
> Hans Verkuil <hverkuil@...all.nl> wrote:
> 
> > In some cases the poll() implementation in a driver has to do different
> > things depending on the events the caller wants to poll for. An example is
> > when a driver needs to start a DMA engine if the caller polls for POLLIN,
> > but doesn't want to do that if POLLIN is not requested but instead only
> > POLLOUT or POLLPRI is requested. This is something that can happen in the
> > video4linux subsystem.
> 
> The change makes sense to me, FWIW.  One bit of trivia I noticed while
> looking at it:
> 
> > @@ -796,7 +792,7 @@ static int do_poll(unsigned int nfds,  struct poll_list *list,
> >  		 * All waiters have already been registered, so don't provide
> >  		 * a poll_table to them on the next loop iteration.
> >  		 */
> > -		pt = NULL;
> > +		pt->qproc = NULL;
> >  		if (!count) {
> >  			count = wait->error;
> >  			if (signal_pending(current))
> 
> The comment at the beginning of this hunk is no longer accurate since the
> poll_table is, indeed, still being supplied.  The previous comment in the
> same function:
> 
> 				/*
> 				 * Fish for events. If we found one, record it
> 				 * and kill the poll_table, so we don't
> 				 * needlessly register any other waiters after
> 				 * this. They'll get immediately deregistered
> 				 * when we break out and return.
> 				 */
> 
> Could also use tweaking.

Indeed! I'll make an RFCv3 tomorrow fixing this.

Thanks for looking at this!

Regards,

	Hans
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