Re: Attempted summary of suspend-blockers LKML thread

From: Rafael J. Wysocki
Date: Sat Aug 07 2010 - 05:03:42 EST

On Saturday, August 07, 2010, david@xxxxxxx wrote:
> On Sat, 7 Aug 2010, Mark Brown wrote:
> > On Fri, Aug 06, 2010 at 04:35:59PM -0700, david@xxxxxxx 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.

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