[PATCH v4 1/6] Documentation: arm: define DT idle states bindings

[Nicolas Pitre]

On Wed, 18 Jun 2014, Santosh Shilimkar wrote:

> On Wednesday 18 June 2014 04:51 PM, Nicolas Pitre wrote:
> > On Wed, 18 Jun 2014, Santosh Shilimkar wrote:
> > 
> >> On Wednesday 18 June 2014 01:36 PM, Lorenzo Pieralisi wrote:
> >> [..]
> >>> +	To correctly specify idle states timing and energy related properties,
> >>> +	the following definitions identify the different execution phases
> >>> +	a CPU goes through to enter and exit idle states and the implied
> >>> +	energy metrics:
> >>> +
> >>> +	..__[EXEC]__|__[PREP]__|__[ENTRY]__|__[IDLE]__|__[EXIT]__|__[EXEC]__..
> >>> +		    |          |           |          |          |
> >>> +
> >>> +		    |<------ entry ------->|
> >>> +		    |       latency        |
> >>> +						      |<- exit ->|
> >>> +						      |  latency |
> >>> +		    |<-------- min-residency -------->|
> >>> +			       |<-------  wakeup-latency ------->|
> >>> +
> >> I don't know the wakeup latency makes much sense and also correct.
> >> Hardware wakeup latency is actually exit latency. Is it for failed
> >> or abort-able ilde case ? We are adding this as a new parameter
> >> at least from idle states perspective. I think we should just
> >> avoid it.
> > 
> > I explained the rationale for this parameter in a previous email but 
> > Lorenzo didn't carry it over. To be clearer, this should be "worst case 
> > wake-up latency".  It is of interest for PMQOS.  This is the maximum 
> > delay that can be expected from the moment a wake-up event is signaled 
> > and the moment the CPU is back operational.  This is more than just exit 
> > latency.  By default this is entry_latency + exit_latency but when there 
> > is an abortable PREP phase then it may be shorter than that.
> > 
> PMQOS angle is right. It is just that the idle code is not
> going to do anything with this value. But I see a value adding it
> instead of some one doing calculation.

The idle code should take it into account when a PMQOS restriction is in 
effect i.e. avoid using those modes whose worst case wake-up latency is 
too large.

And cpuidle is being migrated into the scheduler as we speak.  So some 
of the values there, namely entry_latency and exit_latency (taken 
separately for timing purposes) will be directly used by the scheduler 
to decide which CPU to wake up for example.

So there is fundamentally 4 parameters if we want to comprehensively 
support all pertinent use cases.


Nicolas