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 01:07:22 +0200
From:	Oliver Neukum <oliver@...kum.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	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)

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.

	Regards
		Oliver

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