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: <200906110001.20718.rjw@sisk.pl>
Date:	Thu, 11 Jun 2009 00:01:20 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Oliver Neukum <oliver@...kum.org>
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)

On Wednesday 10 June 2009, Oliver Neukum wrote:
> Am Mittwoch, 10. Juni 2009 21:27:56 schrieb Rafael J. Wysocki:
> > > What happens if the parent's parent is also suspended? It seems to me
> > > that you must code this recursively.
> >
> > Hmm, I thought I did.
> >
> > [Looks]
> >
> > pm_request_resume(dev) will call pm_request_resume(dev->parent), if
> > necessary, and that will call pm_request_resume(dev->parent->parent) and so
> > on.  Each of them will queue a work item and the one for the topmost parent
> > will be queued first.  So, the resume requests for all parents will be
> > executed before the one for the device, due to the fact that the workqueue
> > is singlethread.
> 
> Sneaky, I overlooked that.
> 
> > Well, there is a bug related to it, namely pm_autosuspend() may change the
> > status to RPM_SUSPENDED after pm_request_resume() has changed it to
> > RPM_WAKE, that needs fixing.
> 
> Ok, maybe this is related. You recurse if the parent isn't in RPM_ACTIVE.
> But that is not enough. You must ensure that all the nodes higher up stay
> in RPM_ACTIVE. It seems to me that you must go up until you find an
> active node (or the root) and put it a blocked state.

If you're referring to pm_autoresume(), then this again is tricky.

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.
*However*, there also is pm_resume_sync(), which can take the device directly
from RPM_SUSPENDED to RPM_RESUMING and that may be done in parallel with our
pm_autoresume().  That's why I put the wait_for_completion() in there.

Best,
Rafael
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