linux-kernel - Re: [PATCH 0/8] Suspend block api (version 7)

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Date:	Tue, 18 May 2010 20:52:28 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Kevin Hilman <khilman@...prootsystems.com>
Cc:	Arve Hjønnevåg <arve@...roid.com>,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	Greg KH <gregkh@...e.de>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Brian Swetland <swetland@...gle.com>,
	Daniel Walker <dwalker@...o99.com>,
	"Theodore Ts'o" <tytso@....edu>, Matthew Garrett <mjg@...hat.com>
Subject: Re: [PATCH 0/8] Suspend block api (version 7)

On Tuesday 18 May 2010, Kevin Hilman wrote:
> Arve Hjønnevåg <arve@...roid.com> writes:
> 
> >
> > PM: Opportunistic suspend support.
> >
> > Power management features present in the current mainline kernel are
> > insufficient to get maximum possible energy savings on some platforms,
> > such as Android, because low power states can only safely be entered
> > from idle.  Suspend, in its current form, cannot be used, since wakeup
> > events that occur right after initiating suspend will not be processed
> > until another possibly unrelated event wake up the system again.
> 
> I think the problems with wakeups in the current suspend path need to
> be described in more detail.  In particular, why check_wakeup_irqs()
> is not enough etc.

That one is really easy.: because some (the majority of?) architectures
don't even implement it.
 
> > On some systems idle combined with runtime PM can enter the same power
> > state as suspend, but periodic wakeups increase the average power
> > consumption. Suspending the system also reduces the harm caused by
> > apps that never go idle. On other systems suspend can enter a much
> > lower power state than idle.
> >
> > To allow Android and similar platforms to save more energy than they
> > currently can save using the mainline kernel, we introduce a mechanism
> > by which the system is automatically suspended (i.e. put into a
> > system-wide sleep state) whenever it's not doing useful work, called
> > opportunistic suspend.
> 
> A definition of "useful work" here would provide clarity and would
> also help clarify by what criteria other on-going work is determined
> to be not useful.

Probably "useful" is not the right word here.  I guess it's more like
"work that can be deferred without visible impact on functionality".

Rafael
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