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    SubjectRe: Attempted summary of suspend-blockers LKML thread
    On Wednesday, August 04, 2010, Paul E. McKenney wrote:
    > On Tue, Aug 03, 2010 at 08:57:58PM -0700, Arjan van de Ven wrote:
    > > On Tue, 3 Aug 2010 17:10:15 -0700
    > > "Paul E. McKenney" <> wrote:
    > > >
    > > > OK, I'll bite...
    > > >
    > > > >From an Android perspective, the differences are as follows:
    > > >
    > > > 1. Deep idle states are entered only if there are no runnable
    > > > tasks. In contrast, opportunistic suspend can happen even when there
    > > > are tasks that are ready, willing, and able to run.
    > >
    > > for "system suspend", this is an absolutely valid statement.
    > > for "use suspend as idle state", it's not so clearly valid.
    > > (but this is sort of a separate problem, basically the "when do we
    > > freeze the tasks that we don't like for power reasons" problem,
    > > which in first order is independent on what kind of idle power state
    > > you pick, and discussed extensively elsewhere in this thread)
    > From what I can see, the Android folks are are using "suspend" in
    > the "system suspend" sense.
    > I agree that the proposals for freezing subsets of the tasks in the
    > system are independent of whether idle or suspend is being used.
    > Instead, such freezing depends on (for example) whether or not the
    > display is active.
    > That said, freezing subsets of tasks is a nice-to-have rather than a
    > hard requirement for Android. Though I suspect that the appearance
    > of a reliable way of freezing subsets of tasks just might promote
    > this to a hard requirement. ;-)
    > > > 2. There can be a set of input events that do not bring the
    > > > system out of suspend, but which would bring the system out of a deep
    > > > idle state. For example, I believe that it was stated that
    > > > one of the Android-based smartphones ignores touchscreen input while
    > > > suspended, but pays attention to it while in deep idle states.
    > >
    > > I would argue that this is both a hardware specific issue, but also a
    > > policy issue. From the user point of view, screen off with idle and
    > > screen off with suspend aren't all that different (if my phone would
    > > decide to idle rather than suspend because some app blocks suspend... I
    > > wouldn't expect a difference in behavior when I touch the screen).
    > > "Screen off -> don't honor touch after a bit" is almost an independent,
    > > but very real, policy problem (and a forced one in suspend, I'll grant
    > > you that). I could even argue that the policy decision "we don't care
    > > about the touch screen input" is a pre-condition for entering suspend
    > > (or in android speak, caring for touch screen input/having the touch
    > > screen path active would be a suspend blocker)
    > I agree that the subset of input events that do not bring the system out
    > of suspend would be governed both by hardware capabilities and by policy.
    > > > 3. The system comes out of a deep idle state when a timer
    > > > expires. In contrast, timers cannot expire while the
    > > > system is suspended. (This one is debatable: some people
    > > > argue that timers are subject to jitter, and the suspend
    > > > case for timers is the same as that for deep idle states,
    > > > but with unbounded timer jitter. Others disagree. The
    > > > resulting discussions have produced much heat, but little
    > > > light. Such is life.)
    > >
    > > I'll debate it even harder in that it's platform specific whether
    > > timers can get the system out of suspend or not. Clearly on the Android
    > > platform in question that's not the case, but for some of the Intel
    > > phone silicon for example, timers CAN be wake sources to get you out of
    > > suspend just fine. It just depend on which exact hw you talk about.
    > > Generally, even if the fast timers aren't wake up sources, there'll be
    > > some sort of alarm thing that you can pre-wake.. but yes you are right
    > > in saying that's rather lame.
    > > Either way, it's not a general property of suspend, but a property of
    > > suspend on the specific platform in question.
    > Good point, I do need to emphasize the fact that whether or not timers
    > pull the system out of suspend also depends both on hardware and
    > on policy. So I will change my statement to say something like "The
    > system comes out of a deep idle state when a timer expires. In contrast,
    > timers do not necessarily expire while the system is suspended, depending
    > on both hardware support and platform/application policy."

    That's correct IMO.


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