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    SubjectRe: [PATCH 1/8] PM: Opportunistic suspend support.
    From
    2010/5/25 Dmitry Torokhov <dmitry.torokhov@gmail.com>:
    > On Tue, May 25, 2010 at 04:13:35PM -0700, Arve Hjønnevåg wrote:
    >> 2010/5/25 Dmitry Torokhov <dmitry.torokhov@gmail.com>:
    >> > On Tue, May 25, 2010 at 03:37:48PM -0700, Arve Hjønnevåg wrote:
    >> >> 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...
    >> >>
    >> >
    >> > If your userpsace is that stupid - sure. However, you can:
    >> >
    >> > 1. Notify the suspend manager process that he rest of your userspace is
    >> > busy handling keystrokes so that it does not try to suspend while there
    >> > are events pending.
    >>
    >> You are missing the point. There are no event pending. The kernel
    >> reported the key down event, it was handled, but the keypad driver is
    >> still scanning to see if the user presses another key,
    >
    > Employ reasonable timeout.

    Timeout for what? Stop trying to suspend altogether, stop scanning for
    key changes, or a more "reasonable" poll interval?

    >
    >> or releases the
    >> currently held key.
    >>
    >
    > Userspace consumer should wait for the key release and retract "busy"
    > once event is received and handled.
    >

    No it should not. User-space does not know if the key is coming from a
    keypad driver that needs to actively scan the matrix while keys are
    pressed.

    >> >
    >> > 2. Wait a tiny bit after last application notified you that it finished
    >> > processing events.
    >> >
    >> > So basically the difference is that with in-kernel suspend blockers,
    >> > there is a tiny window where we haven't started the suspend yet but are
    >> > about to the driver has a chance to prevent entire system from starting
    >> > sleep.
    >>
    >> No, the difference is that if a driver needs to prevent suspend for an
    >> extended period of time, you don't have user space continuously
    >> polling to see if it can suspend.
    >
    > Why would a driver, on its own, prevent suspend for extended periods of
    > time? I think that the decision should originate from userspace, kernel
    > is here just to serve the requests.
    >

    A driver prevents suspend if suspend would prevent it from working.
    For instance, the gpio keypad matrix code prevents suspend when a key
    is help down, since it has to activly scan the keypad for changes.
    Only no-keys-pressed versus one-or-more-keys-pressed can be detected
    with an interrupt.

    >>
    >> >
    >> > Without the blocker we may start suspending and will stop midcycle. We
    >> > may be even better off in the end since we could leave some devices
    >> > still powered down after aborting system-wide suspend.
    >> >
    >>
    >> That does not sound right.
    >
    > Why doesn't it? If a device implements runtime PM it may chose remain in
    > powered-down mode even if system is awake.
    >

    If the device implements runtime PM it should already be powered-down
    if it is not in use.

    --
    Arve Hjønnevåg
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