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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Message-Id: <200906111548.33751.rjw@sisk.pl>
Date:	Thu, 11 Jun 2009 15:48:33 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Oliver Neukum <oliver@...kum.org>,
	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 Thursday 11 June 2009, Alan Stern wrote:
> On Thu, 11 Jun 2009, Oliver Neukum wrote:
> 
> > Am Donnerstag, 11. Juni 2009 00:01:20 schrieb Rafael J. Wysocki:
> > > We have queued up resume requests for the device's parent, its parent etc.,
> > > the topmost one goes first.  The workqueue is singlethread, so
> > > pm_autoresume() is going to be run for all parents before the device
> > > itself, so if that were the only resume mechanism, it would be enough to
> > > check if the parent is RPM_ACTIVE.
> > 
> >             A (IDLE)
> >     /                                \
> > B (SUSPENDED)         C (SUSPENDED)
> > 
> > Suppose C is to be resumed. This means first in case of A the request
> > to suspend would be cancelled. Here you drop the locks:
> > 
> > +           && (dev->parent->power.runtime_status == RPM_IDLE
> > +             || dev->parent->power.runtime_status == RPM_SUSPENDING
> > +             || dev->parent->power.runtime_status == RPM_SUSPENDED)) {
> > +               spin_unlock_irqrestore(&dev->power.lock, flags);
> > +               spin_unlock_irqrestore(&dev->parent->power.lock, parent_flags);
> > +
> > +               /* We have to resume the parent first. */
> > +               pm_request_resume(dev->parent);
> > 
> > But after pm_request_resume() returns there's no means to make sure
> > nothing alters it back to RPM_SUSPENDED. The workqueue doesn't help
> > you because you've scheduled nothing by that time. The suspension will
> > work because C is still in RPM_SUSPENDED.
> 
> This is an example where usage counters come in handy.

Do you mean we can count suspend/resume requests for a device?

Rafael
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