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

    On Tue, 8 Dec 2009, Alan Stern wrote:
    > Suppose A and B are unrelated devices and we need to impose the
    > off-tree constraint that A suspends after B.

    Ah. Ok, I can imagine the off-tree constraints, but part of my "keep it
    simple" was to simply not do them. If there are constraints that aren't
    in the topology of the tree, then I simply don't think that async is worth
    it in the first place.

    > You misunderstand. The suspend algorithm will look like this:
    > dpm_suspend()
    > {
    > list_for_each_entry_reverse(dpm_list, dev) {
    > down_write(dev->lock);
    > async_schedule(device_suspend, dev);
    > }
    > }
    > device_suspend(dev)
    > {
    > device_for_each_child(dev, child) {
    > down_read(child->lock);
    > up_read(child->lock);
    > }
    > dev->suspend(dev); /* May do off-tree down+up pairs */
    > up_write(dev->lock);
    > }

    Ok, so the above I think work (and see my previous email: I think
    completions would be workable there too).

    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 also think that you are wrong that the above somehow protects against
    non-topological dependencies. If the device you want to keep delay
    yourself suspending for is after you in the list, the down_read() on that
    may succeed simply because it hasn't even done its down_write() yet and
    you got scheduled early.

    But I guess you could do that by walking the list twice (first to lock
    them all, then to actually call the suspend function). That whole
    two-phase thing, except the first phase _only_ locks, and doesn't do any


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      Last update: 2009-12-08 23:35    [W:0.026 / U:1.088 seconds]
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