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 14:36:30 -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 17:22:06 schrieb Alan Stern:
> > > > 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?
> 
> I am talking about the U1 and U2 feature of USB 3.0.
> 
> Or abstractly any power saving state that does autoresume in hardware.
> In these cases you know that you can enter a powersaving state that
> will add X latency.
> 
> In terms of user space API we'll probably add a way for user space
> to specify how much latency may be added for power management's sake.
> If busses are stacked the "latency budget" has to be handled at core level.
> If furthermore states that allow IO but with additional latency are ignored,
> the budget will be calculated wrongly.

Okay, fine.  What does this have to do with Rafael's work?  Why does 
setting the status to RPM_SUSPENDED even when a device is not in a 
low-power state preclude handling buses that automatically change their 
power state?

I don't see any connection between Rafael's work and managing
latencies, beyond the obvious fact that a device will have a higher
latency when it is suspended than when it isn't.

Alan Stern

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