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 13:38:30 +0000
From:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
To:	Alan Stern <stern@...land.harvard.edu>
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 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.

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.
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