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

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Date:	Wed, 2 Jun 2010 09:00:33 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Arve Hjønnevåg <arve@...roid.com>
cc:	James Bottomley <James.Bottomley@...e.de>,
	Peter Zijlstra <peterz@...radead.org>, tytso@....edu,
	LKML <linux-kernel@...r.kernel.org>,
	Florian Mickler <florian@...kler.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	felipe.balbi@...ia.com, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

On Tue, 1 Jun 2010, Arve Hjønnevåg wrote:
> 2010/6/1 Thomas Gleixner <tglx@...utronix.de>:
> >
> > On Mon, 31 May 2010, Arve Hjønnevåg wrote:
> >
> >> On Mon, May 31, 2010 at 2:46 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> >> > On Mon, 31 May 2010, James Bottomley wrote:
> >> >>
> >> >> For MSM hardware, it looks possible to unify the S and C states by doing
> >> >> suspend to ram from idle but I'm not sure how much work that is.
> >> >
> >> > On ARM, it's not rocket science and we have in tree support for this
> >> > already (OMAP). I have done the same thing on a Samsung part as a
> >> > prove of concept two years ago and it's really easy as the hardware is
> >> > sane. Hint: It's designed for mobile devices :)
> >> >
> >>
> >> We already enter the same power state from idle and suspend on msm. In
> >> the absence of misbehaving apps, the difference in power consumption
> >> is entirely caused by periodic timers in the user-space framework
> >> _and_ kernel. It only takes a few timers triggering per second (I
> >> think 3 if they do no work) to double the average power consumption on
> >> the G1 if the radio is off. We originally added wakelocks because the
> >> hardware we had at the time had much lower power consumption in
> >> suspend then idle, but we still use suspend because it saves power.
> >
> > So how do you differentiate between timers which _should_ fire and
> > those you do not care about ?
> >
> 
> Only alarms are allowed to fire while suspended.
> 
> > We have mechanisms in place to defer timers so the wakeups are
> > minimized. If that's not enough we need to revisit.
> >
> 
> Deferring the the timers forever without stopping the clock can cause
> problems. Our user space code has a lot of timeouts that will trigger
> an error if an app does not respond in time. Freezing everything and
> stopping the clock while suspended is a lot simpler than trying to
> stop individual timers and processes from running.

And resume updates timekeeping to account for the slept time. So the
only way to get away with that is to sleep under a second or just
ignoring the update by avoiding the access to rtc. 

So how do you keep timekeeping happy ?

Thanks,

	tglx