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    SubjectRe: [PATCH 1/8] PM: Opportunistic suspend support.
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    Arve Hjønnevåg <arve@android.com> writes:

    > 2010/5/25 Dmitry Torokhov <dmitry.torokhov@gmail.com>:
    >> On Tue, May 25, 2010 at 03:23:23PM -0700, Arve Hjønnevåg wrote:
    >>> On Tue, May 25, 2010 at 11:47 AM, Dmitry Torokhov
    >>> <dmitry.torokhov@gmail.com> wrote:
    >>> > On Tue, May 25, 2010 at 02:35:17PM -0400, Alan Stern wrote:
    >>> >> On Tue, 25 May 2010, Dmitry Torokhov wrote:
    >>> >>
    >>> >> > > Here's the scenario:
    >>> >> > >
    >>> >> > > The system is awake, and the user presses a key. The keyboard driver
    >>> >> > > processes the keystroke and puts it in an input queue.  A user process
    >>> >> > > reads it from the event queue, thereby emptying the queue.
    >>> >> > >
    >>> >> > > At that moment, the system decides to go into opportunistic suspend.
    >>> >> > > Since the input queue is empty, there's nothing to stop it.  As the
    >>> >> > > first step, userspace is frozen -- before the process has a chance to
    >>> >> > > do anything with the keystroke it just read.  As a result, the system
    >>> >> > > stays asleep until something else wakes it up, even though the
    >>> >> > > keystroke was important and should have prevented it from sleeping.
    >>> >> > >
    >>> >> > > Suspend blockers protect against this scenario.  Here's how:
    >>> >> > >
    >>> >> > > The user process doesn't read the input queue directly; instead it
    >>> >> > > does a select or poll.  When it sees there is data in the queue, it
    >>> >> > > first acquires a suspend blocker and then reads the data.
    >>> >> > >
    >>> >> > > Now the system _can't_ go into opportunistic suspend, because a suspend
    >>> >> > > blocker is active.  The user process can do whatever it wants with the
    >>> >> > > keystroke.  When it is finished, it releases the suspend blocker and
    >>> >> > > loops back to the select/poll call.
    >>> >> > >
    >>> >> >
    >>> >> > What you describe can be done in userspace though, via a "suspend manager"
    >>> >> > process. Tasks reading input events will post "busy" events to stop the
    >>> >> > manager process from sending system into suspend. But this can be confined to
    >>> >> > Android userspace, leaving the kernel as is (well, kernel needs to be modified
    >>> >> > to not go into suspend with full queues, but that is using existing kernel
    >>> >> > APIs).
    >>> >>
    >>> >> I think that could be made to work.  And it might remove the need for
    >>> >> the userspace suspend-blocker API, which would be an advantage.  It
    >>> >> could even remove the need for the opportunistic-suspend workqueue --
    >>> >> opportunistic suspends would be initiated by the "suspend manager"
    >>> >> process instead of by the kernel.
    >>> >>
    >>> >> However you still have the issue of modifying the kernel drivers to
    >>> >> disallow opportunistic suspend if their queues are non-empty.  Doing
    >>> >> that is more or less equivalent to implementing kernel-level suspend
    >>> >> blockers.  (The suspend blocker approach is slightly more efficient,
    >>> >> because it will prevent a suspend from starting if a queue is
    >>> >> non-empty, instead of allowing the suspend to start and then aborting
    >>> >> it partway through.)
    >>> >>
    >>> >> Maybe I'm missing something here...  No doubt someone will point it out
    >>> >> if I am.
    >>> >>
    >>> >
    >>> > Well, from my perspective that would limit changes to the evdev driver
    >>> > (well, limited input core plumbing will be needed) but that is using the
    >>> > current PM infrastructure. The HW driver changes will be limited to what
    >>> > you described "type 2" in your other e-mail.
    >>> >
    >>> > Also, not suspending while events are in progress) is probably
    >>> > beneficial for platforms other than Android as well. So unless I am
    >>> > missing something this sounds like a win.
    >>> >
    >>>
    >>> How would this limit the changes you need in the evdev driver? It need
    >>> to block suspend when there are unprocessed events in some queues.
    >>> Suspend blockers gives you an api to do this, without it, you check
    >>> the queues in your suspend hook and abort suspend if they are not
    >>> empty. Without suspend blockers you have no api to signal that it is
    >>> OK to suspend again, so you are forcing the thread that tried to
    >>> suspend to poll until you stop aborting suspend.
    >>
    >> No, you do not need to poll. You just set a timeout (short or long,
    >> depending on your needs) and if no userspace task blocked suspend
    >> durng that time you attempt to initiate suspend from your manager
    >> process. If it succeeds - good, if not that means that more events came
    >> your way and you have to do it later.
    >>
    >
    > How is that not polling? If the user is holding down a key, the keypad
    > driver has to block suspend, and user space will try to suspend again
    > and again and again...

    Then the userspace suspend manager should be a little more clever
    and should not blindly retry continuously.

    It should be more like a governor which makes some simple decisions
    based on previous events, simple heuristics, uses timeouts etc.,

    Kevin

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