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

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Date:	Thu, 7 Jul 2011 10:42:55 -0600
From:	Jonathan Corbet <corbet@....net>
To:	Hans Verkuil <hverkuil@...all.nl>
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 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.

jon
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