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    SubjectRe: 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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