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