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To: Arve =?iso-8859-1?Q?Hj=F8nnev=E5g?= <arve@android.com>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>, linux-pm@lists.linux-foundation.org,
       linux-kernel@vger.kernel.org, Greg KH <gregkh@suse.de>,
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       Alan Stern <stern@rowland.harvard.edu>,
       Brian Swetland <swetland@google.com>,
       Daniel Walker <dwalker@fifo99.com>, "Theodore Ts'o" <tytso@mit.edu>,
       Matthew Garrett <mjg@redhat.com>
Subject: Re: [PATCH 0/8] Suspend block api (version 7)
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From: Kevin Hilman <khilman@deeprootsystems.com>
Organization: Deep Root Systems, LLC
Date: Tue, 18 May 2010 09:18:12 -0700
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Arve Hjønnevåg <arve@android.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.

> 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.

Kevin