Re: [RFC] lp_events: an lternitive to suspend blocker user mode and kernel API

[Florian Mickler <florian@mickler.org>]

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