Re: [PATCH 2/2] hwmon: (ina3221) Add operating mode support

[Guenter Roeck]

On Wed, Oct 10, 2018 at 05:24:12PM -0700, Nicolin Chen wrote:
> Hi Guenter,
> 
> On Wed, Oct 10, 2018 at 04:43:00PM -0700, Guenter Roeck wrote:
> > > > The effort to do all this using CPU cycles would in most if not all cases
> > > > outweigh any perceived power savings. As such, I just don't see the
> > > > practical use case.
> > > 
> > > It really depends on the use case and how often the one-shot gets
> > > triggered. For battery-powered devices, running in the continuous
> > > mode does consume considerable power based on the measurement from
> > > our power folks. If a system is running in a power sensitive mode,
> > > while it still needs to occasionally check the inputs, it could be
> > > a use case for one-shot mode, though it's purely a user decision.
> > > 
> > That would actually be a use case for runtime power management.
> > The power used by a modern sensor chip is miniscule compared
> > to the power consumed by other components in the system,
> > which may explain why no one bothered looking into runtime
> > power management for sensors.
> > 
> > This is also an argument against any device or subsystem specific solution:
> > Users will want to be able to control power consumption for all devices
> > in the system, not just for sensors. A device specific power control
> > mechanism would, from user space perspective, be a nightmare.
> 
> That makes sense to me. Actually we wanted a solution more on the
> driver side so that user space doesn't need to be involved. But our
> downstream solution (switching different modes when certain number
> of CPUs get hot plugged in/out) wasn't accepted years ago by other
> subsystem maintainers, being commented that it's a user decision,
> IIRC. So I thought that having a sysfs node might be a generic way
> for "user decision". But I guess I should have tried to seek for a
> solution in the kernel like runtime PM as you said.
> 
> > > > power-down mode effectively reinvents runtime power management (runtime
> > > > suspend/resume support) and is thus simply unacceptable.
> > > 
> > > Similar to one-shot, if a system is in a low power mode where it
> > > doesn't want to check the inputs anymore, I feel the user space
> > > could at least make the decision to turn on/off the chips, I am
> > > not quite sure if the generic runtime PM system already has this
> > > kind of support though.
> > > 
> > Please look up "runtime power management". It provides the basic
> > mechanism to turn off / disable a device if it is not used. The point
> > here is that the basic mechanism is there, even though it may not
> > be perfect. If it is not perfect, it needs to be improved.
> > Implementing a per-subsystem alternate method would be the wrong
> > approach.
> 
> I have implemented runtime PM suspend/resume in other drivers but
> what I know is that it relies on someone to call get_sync and put
> functions accordingly and it doesn't seem to have built-in system
> level low-power or power-sensitive mode for those use cases that
> I mentioned previously. But I agree with your point and will take
> a closer look.
> 
> One more question here, and this might sound a bit abuse of using
> the existing hwmon ABI: would it sound plausible to you that the
> driver powers down the chip when all three channels get disabled
> via in[123]_enable nodes? :)
> 

I would not call that an abuse, no.

Guenter