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:	Wed, 10 Jun 2009 19:42:50 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Oliver Neukum <oliver@...kum.org>
cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	<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 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.

Alan Stern

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