linux-kernel - Re: [patch update] Re: [linux-pm] Run-time PM idea (was: Re: [RFC][PATCH 0/2] PM: Rearrange core suspend code)

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Date:	Thu, 11 Jun 2009 11:22:06 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Oliver Neukum <oliver@...kum.org>
cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [patch update] Re: [linux-pm] Run-time PM idea (was: Re:
 [RFC][PATCH 0/2] PM: Rearrange core suspend code)

On Thu, 11 Jun 2009, Oliver Neukum wrote:

> Am Donnerstag, 11. Juni 2009 16:52:03 schrieb Alan Stern:
> > > Under this definition all devices behind an inactive link are suspended,
> > > because they can't do any I/O.  Which appears to makes sense, because
> > > their drivers have to be notified before the link is suspended and the
> > > link has to be turned on for the devices to be able to communicate with
> > > the CPU and RAM.
> > >
> > > If this definition is adopted, then it's quite clear that the device can
> > > only be suspended if all of its children are suspended and it's always
> > > necessary to resume the parent of a device in order to resume the device
> > > itself.
> >
> > Okay, I'll agree to that.  It should be made clear that a device which
> > is "suspended" according to this definition is not necessarily in a
> > low-power state.  For example, before powering down the link to a disk
> > drive you might want the drive's suspend method to flush the drive's
> > cache, but it wouldn't have to spin the drive down.
> 
> This precludes handling busses that have low power states that are
> left automatically. If such links are stacked the management of acceptable
> latencies cannot be left to the busses.
> An actual example are the link states of USB 3.0

I don't understand.  Can you explain more fully?

Alan Stern

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