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:	Fri, 12 Jun 2009 10:32:02 -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 Fri, 12 Jun 2009, Oliver Neukum wrote:

> Am Freitag, 12. Juni 2009 04:16:10 schrieb Alan Stern:
> > What tree constraint?  You mean that the PM core shouldn't allow
> > devices to suspend unless all their children are suspended?  Why
> > doesn't it still apply?
> 
> Because the hardware doesn't need it.

But maybe drivers need it.

> > Remember, when Rafael and I say "suspend" here, we don't mean "go to a
> > low-power state".  We mean "the PM core calls the runtime_suspend
> > method".  No matter what actions the link hardware may decide to take
> > on its own, the PM core will still want to observe the
> > all-children-suspended restriction when calling runtime_suspend
> > methods.
> 
> No. The core if it insists all children be suspended will not use
> the hardware's full capabilities.

That isn't what I said.  The core does not insist that all children be 
suspended, i.e., be in a low-power state.  It insists only that the 
children's drivers' runtime_suspend methods have been called.  Those 
methods are not obligated to put the children in a low-power state.

> If it leaves such power saving measures to the drivers, latency
> accounting will be wrong.
> 
> > > I think there are devices who can be suspended while children are active
> > > and devices which can not be. This is an attribute of the device and
> > > should be evaluated by the core.
> >
> > Clearly it should be decided by the driver.  Should there be a bit for
> > it in the dev_pm_info structure?
> 
> Yes.

That would resolve the issue.

Alan Stern

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