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

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Date:	Fri, 14 May 2010 00:28:49 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Tony Lindgren <tony@...mide.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Paul Walmsley <paul@...an.com>,
	Arve Hjønnevåg <arve@...roid.com>,
	"Linux-pm mailing list" <linux-pm@...ts.linux-foundation.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
	Kevin Hilman <khilman@...prootsystems.com>,
	magnus.damm@...il.com, "Theodore Ts'o" <tytso@....edu>,
	mark gross <mgross@...ux.intel.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Geoff Smith <geoffx.smith@...el.com>,
	Brian Swetland <swetland@...gle.com>,
	Matthew Garrett <mjg@...hat.com>,
	Benoît Cousson <b-cousson@...com>,
	linux-omap@...r.kernel.org, Vitaly Wool <vitalywool@...il.com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Liam Girdwood <lrg@...mlogic.co.uk>
Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 6)

On Friday 14 May 2010, Tony Lindgren wrote:
> * Alan Stern <stern@...land.harvard.edu> [100513 14:56]:
> > On Thu, 13 May 2010, Tony Lindgren wrote:
> > 
> > > > > And that's why 
> > > > > it should be handled by runtime power management instead.
> > > > 
> > > > Runtime PM is not capable of freezing userspace and shutting down CPUs.  
> > > > More or less by definition -- if it could then it wouldn't be "runtime" 
> > > > any more, since the processor wouldn't be running.
> > > 
> > > Not true. We are already powering off CPUs and rebooting them for
> > > at least omaps in every idle loop using cpuidle. The memory stays on.
> > 
> > Okay, that's a valid point.  But is that approach usable in general 
> > (i.e., on non-OMAP systems)?
> 
> Yes if your system wakes to interrupts at least. If your system does
> not wake to timer events, then you'll get missed timers. So it should
> work on PC too with CONFIG_NO_HZ and if RTC was used for the timer
> wake event.
>  
> > How do you handle situations where the CPU is currently idle but an 
> > event (such as I/O completion) is expected to occur in the near future?  
> > You don't want to power-off and reboot then, do you?
> 
> The idle code looks at next_timer_interrupt() value, then if the
> next timer event if far enough ahead, the system powers down and
> wakes to the timer interrupt. It also wakes to device interrupts.

For the record, waking to interrupts doesn't work on quite some systems
(like ACPI-based PCs for one example).

Rafael
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