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

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Date:	Sun, 4 Mar 2012 23:56:17 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Arve Hjønnevåg <arve@...roid.com>
Cc:	Matt Helsley <matthltc@...ibm.com>,
	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>,
	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>,
	jeffbrown@...roid.com
Subject: Re: [RFC][PATCH 4/7] Input / PM: Add ioctl to block suspend while event queue is not empty

On Tuesday, February 28, 2012, Arve Hjønnevåg wrote:
> On Sun, Feb 26, 2012 at 12:57 PM, Rafael J. Wysocki <rjw@...k.pl> 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?
> >
> > Rafael
> >
> > ---
> 
> I don't think it is useful to tie an evdev implementation to epoll
> that way. You just replaced the ioctl with a new control function.
> 
> The code below tries to implement the same flag without modifying
> evdev at all. The behavior of this is different as it will keep the
> device awake until user-space calls epoll_wait again. I also used an
> extra wakeup source to handle the function that runs without the
> spin_lock held which means that non-wakeup files in same epoll list
> could abort suspend.

Well, if that works for you, it will be better than adding ioctls to evdev
(and presumably a number of other devices).

Care to resubmit with a proper changelog and sign-off?

Rafael
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