linux-kernel - Re: [PATCH] epoll: Add a flag, EPOLLWAKEUP, to prevent suspend while epoll events are ready

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Date:	Tue, 17 Jul 2012 07:14:44 +0200
From:	Michael Kerrisk <mtk.manpages@...il.com>
To:	Arve Hjønnevåg <arve@...roid.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>, NeilBrown <neilb@...e.de>,
	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>,
	Alan Stern <stern@...land.harvard.edu>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>, mtk@....net
Subject: Re: [PATCH] epoll: Add a flag, EPOLLWAKEUP, to prevent suspend while
 epoll events are ready

On Tue, Jul 17, 2012 at 12:04 AM, Arve Hjønnevåg <arve@...roid.com> wrote:
> On Mon, Jul 16, 2012 at 4:00 AM, Rafael J. Wysocki <rjw@...k.pl> wrote:
>> On Monday, July 16, 2012, Michael Kerrisk wrote:
>>> Arve, Rafael,
>>>
>>> On Tue, May 1, 2012 at 7:33 AM, Arve Hjønnevåg <arve@...roid.com> wrote:
>>> > When an epoll_event, that has the EPOLLWAKEUP flag set, is ready, a
>>> > wakeup_source will be active to prevent suspend. This can be used to
>>> > handle wakeup events from a driver that support poll, e.g. input, if
>>> > that driver wakes up the waitqueue passed to epoll before allowing
>>> > suspend.
>>>
>>> It's late it the -rc series,
>>
>> Well, exactly. :-)

If someone had CCed linux-api@ along the way (as per
Documentation/SubmitChecklist), it might have helped ;-)

>>
>>> but it strikes me that CAP_EPOLLWAKEUP is
>>> a poor name for the capability that governs the use of EPOLLWAKEUP.
>>> While on the one hand some capabilities are overloaded
>>> (https://lwn.net/Articles/486306/), on the other hand we should avoid
>>> adding individual capabilities for each new API feature (otherwise
>>> capabilities become administratively unwieldy).
>>>
>>> This capability is not really about "EPOLL". It's about the ability to
>>> block system suspend. Therefore, IMO, a better name would be something
>>> like: CAP_BLOCK_SUSPEND. This name is better because there might be
>>> some other API feature that is later added that also has the effect of
>>> preventing system suspends, and we could reasonably govern that
>>> feature with the same capability.
>
> We already have another api, "/sys/power/wake_lock", that allow
> user-space to block suspend. Do we want to apply this capability that
> api as well, or only to apis that do not have other ways to restrict
> access?

Well, the question is: is there a governor on the use of
/sys/power/wake_lock? It makes sense either they are both governed
(preferably by the same mechanism, I would have thought), or neither
is.

>>> Does that seem sensible to you? I can send a patch for the name change.
>>
>> I'm not sure what Arve thinks about that, but I'd be fine with that.
>>
>> Arve, what do you think?
>>
>
> CAP_BLOCK_SUSPEND is fine with me, but if it does not apply to the
> sysfs interface, then the comment should probably mention this.

I've sent a patch, but omitted mention of API details in the comments.
Maybe that can be changed afterward, when a decision has been reached
about governing /sys/power/wake_lock.

Thanks,

Michael

-- 
Michael Kerrisk Linux man-pages maintainer;
http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface", http://blog.man7.org/
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