linux-kernel - Re: Async resume patch (was: Re: [GIT PULL] PM updates for 2.6.33)

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Date:	Wed, 9 Dec 2009 10:49:56 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Zhang Rui <rui.zhang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	pm list <linux-pm@...ts.linux-foundation.org>
Subject: Re: Async resume patch (was: Re: [GIT PULL] PM updates for 2.6.33)

On Wed, 9 Dec 2009, Mark Brown wrote:

> On Tue, Dec 08, 2009 at 09:35:59PM -0500, Alan Stern wrote:
> > On Tue, 8 Dec 2009, Linus Torvalds wrote:
> 
> > > It's just that I think the "looping over children" is ugly, when I think 
> > > that by doing it the other way around you can make the code simpler and 
> > > only depend on the PM device list and a simple parent pointer access.
> 
> > I agree that it is uglier.  The only advantage is in handling
> > asynchronous non-tree suspend dependencies, of which we probably won't
> > have very many.  In fact, I don't know of _any_ offhand.
> 
> There's some potential for this in embedded audio - it wants to bring
> down the entire embedded audio subsystem at once before the individual
> devices (and their parents) get suspended since bringing them down out
> of sync can result in audible artifacts.  Depending on the system the
> suspend may take a noticable amount of time so it'd be nice to be able
> to run it asynchronously, though we don't currently do so.

For something like bringing down the entire embedded audio subsystem, 
which isn't directly tied to a single device, you would probably be 
better off doing it when the PM core broadcasts a suspend notification
(see register_pm_notifier() in include/linux/suspend.h).  This occurs 
before any devices are suspended, so synchronization isn't an issue.

> At the minute we get away with this mostly through not being able to
> represent the cases that are likely to actually trip up over it.
> 
> > Interestingly, this non-tree dependency problem does not affect resume.
> 
> Embedded audio does potentially - the resume needs all the individual
> devices in the subsystem and can take a substantial proportion of the
> overall resume time.  Currently we get away with a combination of
> assuming that all the drivers are live when we decide to start resuming
> them and using the ALSA userspace API to deal with bringing the resume
> out of line, but it's not ideal.

You can do the same thing with the resume notifier.

Alan Stern

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