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

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Message-ID: <20100504145332.GB3651@opensource.wolfsonmicro.com>
Date:	Tue, 4 May 2010 15:53:32 +0100
From:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Matthew Garrett <mjg@...hat.com>,
	Kevin Hilman <khilman@...prootsystems.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Arve Hjønnevåg <arve@...roid.com>,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	Tejun Heo <tj@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
	Paul Walmsley <paul@...an.com>, magnus.damm@...il.com,
	mark gross <mgross@...ux.intel.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Geoff Smith <geoffx.smith@...el.com>
Subject: Re: [PATCH 0/8] Suspend block api (version 6)

On Tue, May 04, 2010 at 09:51:39AM -0400, Alan Stern wrote:
> On Tue, 4 May 2010, Matthew Garrett wrote:
> > On Mon, May 03, 2010 at 04:37:22PM -0700, Kevin Hilman wrote:

> > > Please forgive the ignorance of ACPI (in embedded, we thankfully live
> > > in magical world without ACPI) but doesn't that already happen with
> > > CPUidle and C-states?  I think of CPUidle as basically runtime PM for
> > > the CPU.  IOW, runtime PM manages the devices, CPUidle manages the CPU
> > > (via C-states), resulting in dynaimc PM for the entire system.  What
> > > am I missing?

> > ACPI doesn't provide any functionality for cutting power to most devices 
> > other than shifting into full system suspend. The number of wakeup 
> > events available to us on a given machine is usually small and the 
> > wakeup latency large, so it's not terribly practical to do this 
> > transparently on most hardware.

> Another thing that Kevin is missing: There is more to the system than
> the devices and the CPU.  For example: RAM, an embedded controller (on
> modern desktop/laptop systems), a power supply, and so on.  Dynamic PM
> for the CPU and the devices won't power-down these things, but system
> PM will.

In an embedded system I'd expect that these other system devices would
fall naturally out through the management of the CPUs and devices - for
example, the drivers for the individual devices could use the regulator
API to manage their supplies and runtime PM is being used to manage CPU
core stuff - or could at least readily be handled in a similar fashion.

This isn't to say that we're there yet from an implementation point of
view, of course.
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