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

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

On Mon, Mar 17, 2014 at 01:49:40PM +0000, Antti P Miettinen wrote:
> From: Lorenzo Pieralisi <lorenzo.pieralisi@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@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.

I am not following Antti I am sorry. States are ordered in terms of
power consumption which also means that deeper idle states have a longer
required min-residency to break even against idle_standby in order to actually
save power.

When we make a decision on what idle state to enter all we do, and
that's OS agnostic, is predicting (+checking the next event) the next IRQ and
see if it is worth entering a state or not. We have to compare it against
a baseline, which is the processor being in standbywfi and that's what
these bindings define.

I do not understand why you want to define min-residency against the
previous shallower state.

What this binding says is: standbywfi is the shallower idle state in
power consumption terms. Deeper idle states save more power than
standbywfi if the residency in that state is at least min-residency.

I do not see where the problem is to be honest, maybe I need an example.

Thanks!
Lorenzo