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    SubjectRe: [linux-pm] suspend blockers & Android integration
    On Sun, 6 Jun 2010, Matthew Garrett wrote:

    > The difference between idle-based suspend and opportunistic suspend is
    > that the former will continue to wake up for timers and will never be
    > entered if something is using CPU, whereas the latter will be entered
    > whenever no suspend blocks are held. The problem with opportunistic
    > suspend is that you might make the decision to suspend simultaneusly
    > with a wakeup event being received. Suspend blocks facilitate
    > synchronisation between the kernel and userspace to ensure that all such
    > events have been consumed and handld appropriately.

    Remember that suspend takes place in several phases, the first of which
    is to freeze tasks. The phases can be controlled individually by the
    process carrying out a suspend, and there's nothing to prevent you from
    stopping after the freezer phase. Devices won't get powered down, but
    Android uses aggressive runtime power management for its devices
    anyway.

    If you do this then the synchronization can be carried out entirely
    from userspace, with no need for kernel modifications such as suspend
    blockers. And since Android can reach essentially the same low-power
    state from idle as from suspend, it appears that they really don't need
    any kernel changes at all.

    Alan Stern



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