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    SubjectRe: [RFC] lp_events: an lternitive to suspend blocker user mode and kernel API
    On Mon, 31 May 2010 09:57:53 +1000
    Neil Brown <neilb@suse.de> wrote:

    > On Sun, 30 May 2010 13:04:10 -0700
    > mark gross <640e9920@gmail.com> wrote:
    >
    > > Low Power Events is a possible alternative to suspend blocker / wake
    > > lock API used by Android.
    >
    > Here is how I see your proposal. It is of course possible that I
    > misunderstood bits, so please correct me where I'm wrong.
    >
    > 1/ You have introduced a new mechanism for requesting a transition
    > to a low power state. This involves writing a number to /dev/lpe_enter.
    > It is not clear to me from your text what the magic number really means.
    > I think this parallels writing to /sys/power/state, but achieves the same
    > result though a different mechanism and adds some extra checking.
    > So: I don't understand the numbers, and I don't see why we need a
    > second way to request a low power state. Probably I missed something
    > important.

    I can only think for lpe to provide the levels and have userspace and
    platform code hook into there. Else you would have a dependency from
    userspace to platform code.

    >
    > 2/ Rather than tracking wake-events from the hardware up through possibly
    > several kernel modules, you go directly from hardware to user-space so each
    > event is potentially presented to user-space twice: once as a "wake up
    > from low power state" event and once following the normal path (maybe a
    > key-press event, maybe a serial-port event, maybe a network receive event).
    > I can see that this is a very tempting approach. It allows all those
    > intermediate modules to remain unchanged and that is good.
    > However it isn't clear to me that this would be easy for user-space to use
    > correctly.
    > When an lpe event arrived it would need to wait around for the real event
    > to arrive and then process that. I probably wouldn't wait long, but it
    > would be an indeterminate wait, and it might not be trivial to determine
    > if all events that would cause a wake-up have been consumed as a direct
    > mapping from lpe event to normal event may not always be possible.
    > Maybe this is more of a theoretical problem and in practice it would be
    > easy to get it right - I don't have enough concrete experience to be sure.
    >
    > So: I like the idea of leaving the intermediate layers unchanged, but I'm
    > not convinced it would work.

    To add to this: Is it a correct assumption
    that all wake-up events that leave a driver trickle eventually up to
    userspace?

    I think splitting the actual driver product (i.e. keypress or whatever)
    of a wake-up-event and it's corresponding wake-lock is not possible.
    Because you would have to _somehow_ map the block back to the product
    when you consume the product.

    If you want to abstract the blocking from the kernel-code you probably
    have to introduce an abstract "driver-product" entity where you can do
    all your blocking associated with the product but hidden from the code
    that uses the product. (Which I don't think is feasible, because it
    increases overhead)

    Or am I on the wrong track here?

    cheers,
    Flo


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