Date:	Fri, 21 May 2010 00:21:15 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Arve Hjønnevåg <arve@...roid.com>
Cc:	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	Greg KH <gregkh@...e.de>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Kevin Hilman <khilman@...prootsystems.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 Wednesday 19 May 2010, Arve Hjønnevåg wrote:
...
> 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.  The problem is that to save maximum amount of energy
> all system hardware components need to be in the lowest-power states
> available for as long as reasonably possible, but at the same time the
> system must always respond to certain events, regardless of the
> current state of the hardware.
> 
> The first goal can be achieved either by using device runtime PM and
> cpuidle to put all hardware into low-power states, transparently from
> the user space point of view, or by suspending the whole system.
> However, system suspend, in its current form, does not guarantee that
> the events of interest will always be responded to, since wakeup
> events (events that wake the CPU from idle and the system from
> suspend) that occur right after initiating suspend will not be
> processed until another possibly unrelated event wakes the system up
> again.
> 
> On hardware where idle can enter the same power state as suspend, idle
> combined with runtime PM can be used, but periodic wakeups increase
> the average power consumption. Suspending the system also reduces the
> harm caused by apps that never go idle. There also are systems where
> some devices cannot be put into low-power states without suspending
> the entire system (or the low-power states available to them without
> suspending the entire system are substantially shallower than the
> low-power states they are put into when the entire system is
> suspended), so the system has to be suspended as a whole to achieve
> the maximum energy savings.
> 
> To allow Android and similar platforms to save more energy than they
> currently can save using the mainline kernel, introduce a mechanism by
> which the system is automatically suspended (i.e. put into a
> system-wide sleep state) whenever it's not doing work that's
> immediately useful to the user, called opportunistic suspend.
> 
> For this purpose introduce the suspend blockers framework allowing the
> kernel's power management subsystem to decide when it is desirable to
> suspend the system (i.e. when the system is not doing anything the
> user really cares about at the moment and therefore it may be
> suspended). Add an API that that drivers can use to block
> opportunistic suspend. This is needed to avoid losing wakeup events
> that occur right after suspend is initiated.
> 
> Add /sys/power/policy that selects the behavior of /sys/power/state.
> After setting the policy to opportunistic, writes to /sys/power/state
> become non-blocking requests that specify which suspend state to enter
> when no suspend blockers are active. A special state, "on", stops the
> process by activating the "main" suspend blocker.

That looks good to me.

Thanks,
Rafael
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