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

Alan Stern
2009-12-09T15:49:49Z

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