Re: [PATCH 0/8] Suspend block api (version 6)

[Tony Lindgren <tony@atomide.com>]

* Rafael J. Wysocki <rjw@sisk.pl> [100510 13:27]:
> On Monday 10 May 2010, Kevin Hilman wrote:
> > Hello,
> 
> Hi Kevin,
> 
> > I think many folks are still confused about exactly the problem being
> > solved by this series as well as mixed up between opportunistic
> > suspend and suspend blockers.  Also, how this series impatcs the rest
> > of the kernel (especially PM-aware drivers and subsystems) has caused
> > a bit of confusion.
> > 
> > To help with the confusion, I think a much clearer description of the
> > problem being solved and the proposed solution is needed.  
> > 
> > To that end, I created a starting point for that below which
> > summarizes how I understand the problem and the proposed solution, but
> > of course this should be filled out in more detail and updated as part
> > of the documentation that goes with this series.
> > 
> > Hope this helps improve the understanding of this feature,
> 
> Yes, I think this is helpful.
> 
> > Table of Contents
> > =================
> > 1 Problem Statement 
> > 2 Solution: Opportunistic suspend 
> >     2.1 When to use a suspend blocker 
> >     2.2 Usage in PM aware drivers 
> > 
> > 
> > 1 Problem Statement 
> > ~~~~~~~~~~~~~~~~~~~~
> > 
> > Want to to hit deep power state, even when the system is not actually
> > idle.
> > 
> > Why?
> > 
> > - some hardware is not capable of deep power states in idle
> > - difficulty getting userspace and/or kernel to be idle
> > 
> > 2 Solution: Opportunistic suspend 
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > 
> > Create an additional "idle path" which has new rules for determining
> > idleness.  When this new "idle" is reached, trigger full-system
> > suspend.  Since a suspend is triggered whenever the opportunity
> > arises, this is called opportunistic suspend.

I agree, this is is the right way to handle when to enter suspend.

Especially if the system needs to run while waiting for something
to happen before being able to suspend.

> > The new rules for making the idleness decision are simple:
> > 
> > 1.  system may suspend if and only if no suspend blockers are held

To me it sounds like this should only be allowed to happen when you do:

# echo 1 > /sys/power/suspend_while_idle

As it kills the timers and leads to non-standard behaviour of the apps
as they won't run :)

And then the remaining question is how to make sure the use cases
below can be handled in a clean way.
 
> > 2.1 When to use a suspend blocker 
> > ==================================
> > 
> > [A list of reasons why suspend blockers might be used would be very
> >  helpful here.] 
> > 
> > - ensure wakeup events propagate to userspace (e.g. keypad example in doc)
> > 
> > - screen is on
> > 
> > - someone mentioned "Google use cases"
> >   (would be nice to hear about more of these)
> 
> Yes, I think the Android developers know quite a few cases where suspend
> blockers are useful.
> 
> > 2.2 Usage in PM aware drivers 
> > ==============================
> > 
> > [An example of how a driver already using runtime PM would use 
> >  a suspend blocker would also be helpful.
> 
> When we have any drivers using both in the tree, they will be used as examples
> here.
> 
> Thanks,
> Rafael
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