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

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Date:	Mon, 31 May 2010 10:05:45 +0200
From:	Florian Mickler <florian@...kler.org>
To:	Florian Mickler <florian@...kler.org>
Cc:	linux-kernel@...r.kernel.org, markgross@...gnar.org,
	640e9920@...il.com, Thomas Gleixner <tglx@...utronix.de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Brian Swetland <swetland@...gle.com>,
	Arve Hjønnevåg <arve@...roid.com>,
	linux-pm@...ts.linux-foundation.org,
	"Rafael J. Wysocki" <rjw@...k.pl>, mark.gross@...el.com
Subject: Re: [RFC] lp_events: an lternitive to suspend blocker user mode and
 kernel API

On Mon, 31 May 2010 08:43:56 +0200
Florian Mickler <florian@...kler.org> wrote:

> On Mon, 31 May 2010 09:57:53 +1000
> Neil Brown <neilb@...e.de> wrote:
> 
> > 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? 

I just realized, that you can cancel lpe_blocks via delete_lpe_block(),
so this is not an issue at all.
They can be used just like suspend blockers. 

Also the mapping of lpe_block to "wake event" is the same problem as
with the suspend_blockers... 

So I don't think this is a bad idea after all. It decouples the
suspend_blockers from "suspend" quite nicely. 
Although it still is only "block" or "no block" and not, as was
suggested some sort of more fine grained requirement definition.

Cheers,
Flo
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