Re: [PATCH 1/8] PM: Opportunistic suspend support.

[Florian Mickler <florian@mickler.org>]

On Wed, 26 May 2010 10:42:22 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, 2010-05-25 at 15:33 -0700, Arve Hjønnevåg wrote:
> > The biggest problem here is not that it is hard to change our
> > user-space, but that the proposed change is inferior to what we have
> > now. It forces us to poll until all drivers stop aborting suspend. On
> > one hand we have people telling us that all code that polls is broken
> > and must be fixed (instead of suspending to limit the damage), and on
> > the other hand we have people suggesting we implement opportunistic
> > suspend by polling from user-space until suspend succeeds. 
> 
> No it does _not_. You're really not getting that Dmitry is proposing.
> 
> 
> So your proposal is that when we wake userspace, it
> opens /dev/suspend_blocker _before_ it consumes whatever event, consumes
> the event, deals with the event, then closes the suspend_blocker. Then
> the kernel, upon reaching a 0 suspend_blocker count, will try to suspend
> again.
> 
> 
> What Dmitry proposes is that, the app _before_ it consumes the event,
> pokes at this suspend manager, it increases a blocker count, then
> consumes the event (the kernel will _not_ auto-suspend), handles it and
> then again pokes the suspend manager, this time decreasing the blocker
> count.
> 
> The suspend manager will, upon reaching a 0 block count, suspend the
> machine. If that fails, it means there's something to do, an app will
> inc, work, dec its count, and it will try again once it reaches 0 again.
> 
> There is no polling what-so-ever in this model.
> 
> The only thing is that the kernel will not try to auto-suspend and there
> is no user-space suspend blocker API.

There is polling, because the suspend manager in userspace doesn't have
the whole picture. i.e. it doesn't know if a suspend will be
successfull. 
So for aggressive suspending as a powersave-feature you need to poll
(i.e. retry upon failure). because you don't want to stay unsuspended. 

This discussion really is going in circles. You could all may well be
just posting references to postings by now, instead of actually writing
mails.

Cheers,
Flo