linux-kernel - Re: [RFC][PATCH 4/7] Input / PM: Add ioctl to block suspend while event queue is not empty

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Message-Id: <201202280217.39661.rjw@sisk.pl>
Date:	Tue, 28 Feb 2012 02:17:39 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Matt Helsley <matthltc@...ibm.com>
Cc:	Linux PM list <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Magnus Damm <magnus.damm@...il.com>, markgross@...gnar.org,
	Matthew Garrett <mjg@...hat.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	John Stultz <john.stultz@...aro.org>,
	Brian Swetland <swetland@...gle.com>,
	Neil Brown <neilb@...e.de>,
	Alan Stern <stern@...land.harvard.edu>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>
Subject: Re: [RFC][PATCH 4/7] Input / PM: Add ioctl to block suspend while event queue is not empty

On Monday, February 27, 2012, Matt Helsley wrote:
> On Sun, Feb 26, 2012 at 09:57:18PM +0100, Rafael J. Wysocki wrote:
> > On Friday, February 24, 2012, Matt Helsley wrote:
> > > On Wed, Feb 22, 2012 at 12:34:58AM +0100, Rafael J. Wysocki wrote:
> > > > From: Arve Hjønnevåg <arve@...roid.com>
> > > > 
> > > > Add a new ioctl, EVIOCSWAKEUPSRC, to attach a wakeup source object to
> > > > an evdev client event queue, such that it will be active whenever the
> > > > queue is not empty.  Then, all events in the queue will be regarded
> > > > as wakeup events in progress and pm_get_wakeup_count() will block (or
> > > > return false if woken up by a signal) until they are removed from the
> > > > queue.  In consequence, if the checking of wakeup events is enabled
> > > > (e.g. throught the /sys/power/wakeup_count interface), the system
> > > > won't be able to go into a sleep state until the queue is empty.
> > > > 
> > > > This allows user space processes to handle situations in which they
> > > > want to do a select() on an evdev descriptor, so they go to sleep
> > > > until there are some events to read from the device's queue, and then
> > > > they don't want the system to go into a sleep state until all the
> > > > events are read (presumably for further processing).  Of course, if
> > > > they don't want the system to go into a sleep state _after_ all the
> > > > events have been read from the queue, they have to use a separate
> > > > mechanism that will prevent the system from doing that and it has
> > > > to be activated before reading the first event (that also may be the
> > > > last one).
> > > 
> > > I haven't seen this idea mentioned before but I must admit I haven't
> > > been following this thread too closely so apologies (and don't bother
> > > rehashing) if it has:
> > > 
> > > Could you just add this to epoll so that any fd userspace chooses would be
> > > capable of doing this without introducing potentially ecclectic ioctl
> > > interfaces?
> > > 
> > > struct epoll_event ev;
> > > 
> > > epfd = epoll_create1(EPOLL_STAY_AWAKE_SET);
> > > ev.data.ptr = foo;
> > > epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev);
> > > 
> > > Which could be useful because you can put one epollfd in another's epoll
> > > set. Or maybe as an EPOLLKEEPAWAKE flag in the event struct sort of like
> > > EPOLLET:
> > > 
> > > epfd = epoll_create1(0);
> > > ev.events = EPOLLIN|EPOLLKEEPAWAKE;
> > > epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &ev);
> > 
> > Do you mean something like the patch below, or something different?
> 
> Yeah, this was sort of what I was thinking of. It nicely avoids the
> ioctl() bits. I guess my only issue is the fop mimics the epoll
> interface -- should it just be an fop to manage the file as a wakeup
> source rather than a generic hook into epoll?

I'm not exactly sure what you mean, could you be a bit more specific, please?

Rafael
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