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

[Lorenzo Pieralisi]

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