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

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Message-Id: <201008071101.25384.rjw@sisk.pl>
Date:	Sat, 7 Aug 2010 11:01:24 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	david@...g.hm
Cc:	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Brian Swetland <swetland@...gle.com>,
	kevin granade <kevin.granade@...il.com>,
	"Arve Hj?nnev?g" <arve@...roid.com>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	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 Saturday, August 07, 2010, david@...g.hm wrote:
> On Sat, 7 Aug 2010, Mark Brown wrote:
> 
> > On Fri, Aug 06, 2010 at 04:35:59PM -0700, david@...g.hm wrote:
> >> On Fri, 6 Aug 2010, Paul E. McKenney wrote:
... 
> What we want to have happen in an ideal world is
> 
> when the storage isn't needed (between reads) the storage should shutdown 
> to as low a power state as possible.
> 
> when the CPU isn't needed (between decoding bursts) the CPU and as much of 
> the system as possible (potentially including some banks of RAM) should 
> shutdown to as low a power state as possible.

Unfortunately, the criteria for "not being needed" are not really
straightforward and one of the wakelocks' roles is to work around this issue.

> today there are two ways of this happening, via the idle approach (on 
> everything except Android), or via suspend (on Android)
> 
> Given that many platforms cannot go to into suspend while still playing 
> audio, the idle approach is not going to be able to be eliminated (and in 
> fact will be the most common approach to be used/deugged in terms of the 
> types of platforms), it seems to me that there may be a significant amount 
> of value in seeing if there is a way to change Android to use this 
> approach as well instead of having two different systems competing to do 
> the same job.

There is a fundamental obstacle to that, though.  Namely, the Android
developers say that the idle-based approach doesn't lead to sufficient energy
savings due to periodic timers and "polling applications".  Technically that
boils down to the interrupt sources that remain active in the idle-based case
and that are shut down during suspend.  If you found a way to deactivate all of
them from the idle context in a non-racy fashion, that would probably satisfy
the Android's needs too.

Thanks,
Rafael
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