linux-kernel - Re: [RFD] Automatic suspend

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Message-Id: <200902180026.55533.rjw@sisk.pl>
Date:	Wed, 18 Feb 2009 00:26:53 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Pavel Machek <pavel@....cz>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	mark gross <mgross@...ux.intel.com>,
	"Woodruff, Richard" <r-woodruff2@...com>,
	Uli Luckas <u.luckas@...d.de>,
	Igor Stoppa <igor.stoppa@...ia.com>,
	Brian Swetland <swetland@...gle.com>,
	Len Brown <lenb@...nel.org>
Subject: Re: [RFD] Automatic suspend

On Tuesday 17 February 2009, Alan Stern wrote:
> On Tue, 17 Feb 2009, Rafael J. Wysocki wrote:
> 
> > On Monday 16 February 2009, Alan Stern wrote:
> > > On Mon, 16 Feb 2009, Rafael J. Wysocki wrote:
> > > 
> > > > OK, so I think there are two things that user space may be allowed to do as
> > > > far as putting devices into low power states is concerned:
> > > > * disable/enable the automatic power management of the device (provided that
> > > >   the driver supports the automatic PM)
> > > 
> > > Set the automatic PM parameters (idle timeout, state to go to, etc.).  
> > 
> > Yes.  I'm not sure about the state part, though.
> 
> Maybe, maybe not.  IMO it's too early to tell whether anyone will need 
> this ability, so we shouldn't rule it out.
> 
> > > What about situations where we want to distinguish between the power 
> > > state of the device itself and the power state of the link?  For a disk 
> > > drive we may want to power the link on and off quite a lot, as that 
> > > has low latency, but spinning the disk up and down takes a long time 
> > > and so should have a longer idle-time value.
> > 
> > Well, I'm not sure at the moment.
> > 
> > Do you have any suggestions?
> 
> Not very well fleshed-out ones.  I've got a vague idea for allowing a
> disk to have a 3-level power arrangement: full power, link disabled but
> drive still spinning, and device suspended.

I was thinking about that too.

> Arranging for automatic transitions among those states will be a little
> clumsy but it can be done.  As an example of the clumsiness, this scheme
> requires that the drive has _two_ idle-timeout values, one for the link and
> one for the drive itself.

Well, this generally is the case for PCI devices supporting more than two
power states.

> Another possibility is to set up independent runtime PM for the
> transport and the device.  This means allowing the possibility that the
> transport is suspended while its child (the device) is not.  This is a
> little simpler (there's only one idle-timeout per device, since the
> link is treated as an independent device), but it violates the
> principle of never suspending a parent while there is an active child.

Well, I think the first approach would be better.

Thanks,
Rafael
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