linux-kernel - Re: [linux-pm] [PATCH 0/8] Suspend block api (version 6)

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Date:	Mon, 17 May 2010 22:22:22 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Brian Swetland <swetland@...gle.com>
Cc:	me@...ipebalbi.com, James Bottomley <James.Bottomley@...e.de>,
	Kevin Hilman <khilman@...prootsystems.com>,
	Alan Stern <stern@...land.harvard.edu>,
	linux-omap@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>,
	Geoff Smith <geoffx.smith@...el.com>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Tejun Heo <tj@...nel.org>,
	"Linux-pm mailing list" <linux-pm@...ts.linux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Liam Girdwood <lrg@...mlogic.co.uk>,
	Matthew Garrett <mjg@...hat.com>
Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 6)

On Monday 17 May 2010, Brian Swetland wrote:
> On Mon, May 17, 2010 at 11:39 AM, Felipe Balbi <me@...ipebalbi.com> wrote:
...
> > but can anyone write an app that holds a suspend_blocker ?? If so, then
> > your goal is already broken, right ? I mean, if anyone can keep a
> > suspend_blocker held forever, you'll never ever sleep, right ? While
> > with runtime, if you keep the keypad open, only the keypad and the paths
> > directly related to it (probably the i2c controller and the power domain
> > where the i2c controller sits) will be kept alive, no ?
> 
> No, you'll never suspend, which is different from never going to the
> lowest CPU power state.  On shipping Android devices we aggressively
> completely power down the CPU in idle whenever we can (based on
> latency requirements generally).  We power off peripherals whenever
> they're not in use.
> 
> This is why I've stated previously that I don't think runtime PM and
> opportunistic suspend are competitive features.

Agreed.

> Everyone who cares about minimizing power should want runtime pm or at least
> similar functionality (our drivers have always powered down peripherals when
> not in use, even while the device is open, etc, prior to the existence
> of runtime PM).

Yes.

> If your environment is such that going to full suspend will not gain
> you anything, then don't use opportunistic suspend.

Exactly.

> We find that there are savings to be had with this model in Android which is
> why we use it.  If you are going to use opportunistic suspend,
> suspend_blockers provide useful functionality.

And as I said, I regard this as a legitimate approach to power management.

Thanks,
Rafael
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