linux-kernel - Re: Attempted summary of suspend-blockers LKML thread

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Message-ID: <AANLkTik2VJewvER0WPwb5vKELq+S8_xqPsAiidc1aRCD@mail.gmail.com>
Date:	Thu, 5 Aug 2010 07:29:51 -0700
From:	Brian Swetland <swetland@...gle.com>
To:	david@...g.hm
Cc:	Matthew Garrett <mjg59@...f.ucam.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	pavel@....cz, florian@...kler.org, stern@...land.harvard.edu,
	peterz@...radead.org, tglx@...utronix.de, alan@...rguk.ukuu.org.uk
Subject: Re: Attempted summary of suspend-blockers LKML thread

On Thu, Aug 5, 2010 at 7:22 AM,  <david@...g.hm> wrote:
>
> Ok, it is now sounding to me like there are two different (but somewhat
> related) purposes that wakelocks are being used for
>
> 1. deciding if the system should go to sleep now or not (what most of the
> discussion has been about)
>
> 2. narrowing the race between going to sleep and wakeup events.
>
> I'm not sure it's possible to completely eliminate the race, even with
> wakelocks there is some time between the time you last check if the wakelock
> is set and when the hardware finishes responding to your commands to go to
> sleep (Unless you can set a level-based interrupt that will wake you up as
> soon as you finish going to sleep)

The transition into sleep is race-free in the wakelock model -- either
the wakeup event happens during the kernel suspend (and grabs a
wakelock), in which case suspend is aborted (and not attempted again
until there are once again no more wakelocks held), or the system
fully suspends to its lowest power mode until a wakeup event brings it
back out again.  Entry to lowest power mode must abort if a wakeup
event/interrupt occurs while it's in process -- exactly how the
handoff happens here is hardware dependent but in practice this is
doable on just about any modern system.

Brian
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