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

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Date:	Fri, 14 May 2010 22:47:01 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Magnus Damm <magnus.damm@...il.com>
cc:	Tony Lindgren <tony@...mide.com>, 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>,
	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>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Matthew Garrett <mjg@...hat.com>,
	Benoît Cousson <b-cousson@...com>,
	<linux-omap@...r.kernel.org>, Vitaly Wool <vitalywool@...il.com>,
	Linus Walleij <linus.walleij@...csson.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 Fri, 14 May 2010, Magnus Damm wrote:

> I agree with you Tony. I thought shutting down CPUs for power
> managment purposes could be done without freezing user space. At least
> that's what we do today with SH-Mobile.

If you can shut down and restart CPUs in the time periods between 
transitions into and out of the idle loop, then yes -- userspace 
doesn't need to be suspended.  After all, if the idle loop is running 
then no user threads are runnable.  (Not necessarily true on SMP 
systems, but you know what I mean.)

> Still not sure how the system wide suspend is different from Runtime
> PM and CPUidle from the hardware perspective...

For embedded systems, apparently the difference is minimal.  For other
systems, like ACPI-based PCs, there is a big difference: Powering down
devices and CPUs still leaves large parts of the system running.

In addition, these larger systems generally don't have aggressive 
runtime PM support, so a significant fraction (maybe more than 50%) of 
devices won't be powered down when they are idle -- whereas system 
suspend powers virtually everything down.

Alan Stern

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