[PATCH RFC v4 3/3] Documentation: arm: define DT idle states bindings

[Antti P Miettinen]

From: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
> Hi Antti,
> 
> On Mon, Mar 17, 2014 at 11:15:07AM +0000, Antti P Miettinen wrote:
>> Sorry for having been lazy in commenting..
> 
> No worries, comments always welcome.
> 
>> From: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
>> Date: Tue, 18 Feb 2014 11:47:31 +0000
>> > +	- min-residency
>> > +		Usage: Required
>> > +		Value type: <prop-encoded-array>
>> > +		Definition: u32 value representing time in microseconds
>> > +			    required for the CPU to be in the idle state to
>> > +			    break even in power consumption terms compared
>> > +			    to idle state idle_standby ([4][5]).
>> 
>> To me this continues to be a bit illdefined. Say we have three states:
>> 0,1,2. State 0 is the idle_standby. Providing a minimum residency for
>> state 1 compared to state 0 sort of makes sense, but if we provide a
>> minimum residency for state 2 compared to state 0 the break even time
>> is going to be smaller than break even when comparing state 1 and
>> state 2. With this data we'd enter state 2 when we'd be better off
>> entering state 1.
> 
> I am not sure I got your reply right, but min-residency for
> state 2 will be higher than state 1, since it has to cater for the
> dynamic power consumed by entering the state (but burns less power
> than state 1 when _in_ the state).
> 
> Entering a state has a power cost and min-residency should take that into
> account, worst-case as per other stats.
> 
> min-residency (and so the break-even) should take into account that
> entering the state is not for free.
> 
> I think that comparing against idle_standby is the only sane way we can
> define that parameter, either that or we remove it.
> 
> Does it make sense ?
> 
> Thanks !
> Lorenzo

The point is that if you compare breakeven between state 0 and state 2
the breakeven time will be smaller that when you compare the breakeven
between state 1 and state 2. Assuming states ordered by "deepness" in
the sense that deeper states have lower in-state power and longer
entry/exit times.

I guess you could specify that the min-residency defines the time when
the state breaks even compared to the previous (shallower) state.

	--Antti