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    SubjectRe: [RFC][PATCH] PM: Avoid losing wakeup events during suspend
    On Mon, 21 Jun 2010, Florian Mickler wrote:

    > > In the end you would want to have communication in both directions:
    > > suspend blockers _and_ callbacks. Polling is bad if done too often.
    > > But I think the idea is a good one.
    > Actually, I'm not so shure.
    > 1. you have to roundtrip whereas in the suspend_blocker scheme you have
    > active annotations (i.e. no further action needed)

    That's why it's best to use both. The normal case is that programs
    activate and deactivate blockers by sending one-way messages to the PM
    process. The exceptional case is when the PM process is about to
    initiate a suspend; that's when it does the round-trip polling. Since
    the only purpose of the polling is to avoid a race, 90% of the time it
    will succeed.

    > 2. it may not be possible for a user to determine if a wake-event is
    > in-flight. you would have to somehow pass the wake-event-number with
    > it, so that the userspace process could ack it properly without
    > confusion. Or... I don't know of anything else...
    > 1. userspace-manager (UM) reads a number (42).
    > 2. it questions userspace program X: is it ok to suspend?
    > [please fill in how userspace program X determines to block
    > suspend]
    > 3a. UM's roundtrip ends and it proceeds to write "42" to the
    > kernel [suspending]
    > 3b. UM's roundtrip ends and it aborts suspend, because a
    > (userspace-)suspend-blocker got activated
    > I'm not shure how the userspace program could determine that there is a
    > wake-event in flight. Perhaps by storing the number of last wake-event.
    > But then you need per-wake-event-counters... :|

    Rafael seems to think timeouts will fix this. I'm not so sure.

    > Do you have some thoughts about the wake-event-in-flight detection?

    Not really, except for something like the original wakelock scheme in
    which the kernel tells the PM core when an event is over.

    Alan Stern

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