linux-kernel - Re: [patch update 3] PM: Introduce core framework for run-time PM of I/O devices

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Message-ID: <Pine.LNX.4.44L0.0906241604510.22054-100000@iolanthe.rowland.org>
Date:	Wed, 24 Jun 2009 16:19:14 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	Oliver Neukum <oliver@...kum.org>,
	Magnus Damm <magnus.damm@...il.com>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	ACPI Devel Mailing List <linux-acpi@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>, Greg KH <gregkh@...e.de>
Subject: Re: [patch update 3] PM: Introduce core framework for run-time PM
 of I/O devices

On Wed, 24 Jun 2009, Rafael J. Wysocki wrote:

> > The difficulty is that some USB interface drivers require remote wakeup
> > to be enabled while their interfaces are suspended.  But remote wakeup
> > is a global setting; it doesn't take effect until the entire physical
> > device is suspended.  (To put it another way, USB has no notion of
> > suspending interfaces.)  This means we must not allow these interfaces
> > to be suspended before the whole device is.  But the whole device is
> > the parent of the interfaces -- if we can't suspend the children before
> > suspending the parent then we're stuck.
> 
> Not if we use the power.ignore_children flag on the parent.
> 
> > Clearly this is something the USB stack has to deal with; it shouldn't
> > affect the general PM framework.  However the only solution I can think
> > of involves subverting the framework, which isn't very nice.  The idea
> > is to ignore runtime_suspend callbacks for these interface drivers;
> > allow them to keep on running even though the PM core thinks they are
> > suspended.  Then suspend and resume them as part of the callbacks for
> > the entire device.  (For interface drivers that don't require remote
> > wakeup there is no problem; it doesn't matter when they get suspended.)
> > 
> > This will work, but it's a hack.  Does anybody have a better idea?
> 
> Well, as I said above, you can set power.ignore_children on the device
> and then it can be suspended even if the interfaces aren't.

Hmm.  The hard part still remains: to make sure that the interfaces 
don't get suspended without the device also getting suspended.

I suppose we could attack this by making the device do a runtime_get on
each of the interfaces, which would be released in the device's
runtime_suspend method.  But then conversely, each interface driver
would have to do its gets and puts on the _device's_ resume_counter.  
If they used the interface counters then the values would never go to 0
and so nothing would ever be suspended.

You've got to admit, this does sound rather bizarre.  :-)  But it ought 
to work...

Alan Stern

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