Re: [RFC PATCH] poll: add poll_requested_events() function

[Jonathan Corbet <corbet@lwn.net>]

On Fri, 1 Jul 2011 15:37:30 +0200
Hans Verkuil <hverkuil@xs4all.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