linux-kernel - Re: [patch update] PM: Introduce core framework for run-time PM of I/O devices (rev. 6)

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Date:	Mon, 29 Jun 2009 18:00:43 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	Greg KH <gregkh@...e.de>, LKML <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [patch update] PM: Introduce core framework for run-time PM of
 I/O devices (rev. 6)

On Mon, 29 Jun 2009, Rafael J. Wysocki wrote:

> > Another possible approach you could take when the call to
> > cancel_delayed_work fails (which should be rare) is to turn on RPM_WAKE
> > in addition to RPM_IDLE and leave the suspend request queued.  When
> > __pm_runtime_suspend sees both flags are set, it should abort and set
> > the status directly back to RPM_ACTIVE.  At that time the idle
> > notifications can start up again.
> > 
> > Is this any better?  I can't see how drivers would care, though.
> 
> There still is the problem that the suspend request is occupying the
> work_struct which cannot be used for any other purpose.

What other purpose?  We don't send idle notifications in RPM_IDLE and
resume requests don't need to be stored since (as described above) they
just set the RPM_WAKE flag.  Hence nothing else needs to use the
work_struct.

>  I don't think this
> is avoidable, though.  This way or another it is possible to have two requests
> pending at a time.
> 
> Perhaps the simplest thing to do would be to simply ignore pending suspend
> requests in both pm_request_resume() and pm_runtime_resume() and to allow
> them to be scheduled at any time.  That shouldn't hurt anything as long as
> pm_runtime_suspend() is smart enough, but it has to be anyway, because it
> can be run synchronously at any time.
> 
> The only question is what pm_runtime_suspend() should do when it sees a pending
> suspend request and quite frankly I think it can just ignore it as well,
> leaving the RPM_IDLE bit set.  In which case the name RPM_IDLE will not really
> be adequate, so perhaps it can be renamed to RPM_REQUEST or something like
> this.
> 
> Then, we'll need a separate work structure for suspend requests, but I have no
> problem with that.

You seem to be thinking about these requests in a very different way
from me.  They don't form a queue or anything like that.  Instead they
mean "Change the device's power state to this value as soon as
possible" -- and they are needed only because sometimes (in atomic or
interrupt contexts) the change can't be made right away.

That's why it doesn't make any sense to have both a suspend and a 
resume request pending at the same time.  It would mean the driver is 
telling us "Change the device's power state to both low-power and 
full-power as soon as possible"!

We should settle on a general policy for how to handle it when a 
driver makes the mistake of telling us to do contradictory things.  
There are three natural policies:

	The first request takes precedence over the second;

	The second request takes precedence over the first;

	Resumes take precedence over suspends.

Any one of those would be acceptable.

Alan Stern

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