Re: [PATCH] iio: humidity: hts221: add power management support

[Brian Masney <masneyb@onstation.org>]

On Thu, May 11, 2017 at 09:35:12AM +0000, Lorenzo BIANCONI wrote:
> > If desired, one way to get rid of the enabled flag would be to
> > add support for runtime power management to automatically shutdown
> > the chip after a period of inactivity. See
> > https://lkml.org/lkml/2017/4/25/101 for an example.
> > 
> 
> I am not a pm_runtime expert but according to the documentation runtime_suspend
> callback is called when device's usage counter and counter of 'active' children
> of the device are equal to 0. Moreover device possible states are 'disabled'
> (HTS221_REG_CNTRL1_ADDR register set to 0) or 'active' (HTS221_REG_CNTRL1_ADDR
> register configured with a given sample rate). In the first condition
> runtime_suspend() will not take any effect whereas in the latter one the
> callback will not be called since device's usage counter is grater than 0.
> Moreover implement system-wide pm support through runtime pm support just to
> avoid a boolean flag seems a little bit overkill to me. What do you think?
> Is my understanding correct?

Sorry, I should have also said that I didn't think that runtime PM was
absolutely required. I also agree adding runtime PM just to remove the
flag is overkill. Runtime PM is nice to have if this sensor is hooked
up to something that may run off a battery (such as a weather station)
to help conserve power. The flag removal is a by product of this. :)

As for the runtime PM reference count, when your driver is initialized,
keep the device off so that the device usage count is initially zero.
Wrap your _write_raw() and _read_raw() functions with the runtime PM
calls and the device will power on and off as needed.

Brian