phosphor-hwmon funcitonal interface

[Matthew Barth]

On 1/14/19 9:33 PM, Lei YU wrote:
>> Thoughts?  (IIRC< I've mentioned this idea before, but now i'm setting
>> up some cycles to hammer it out, as I continue to implement unit-tests
>> in phosphor-hwmon)
> 
> I second this proposal.
> 
> Be noted that this "functional" property on sensor is not the same one on
> inventory, and from my understanding, this functional property is not being
> used by other services (yet), it's just used to check if hwmon should read the
> value for the sensor or not.
> 
Actually, phosphor-fan-presence/control is using the functional property 
that's currently being created where the 'fault' file exists in sysfs 
for the OCCs.
> So we could re-use this property to indicate if a sensor is functional or not,
> especially useful for fans that will get reading error when it's turned off,
> e.g. the fans using Aspeed fan/pwm driver.
> 
This case seems to not follow whether a fan is functional or not when 
its turned off. The fan is still functional, just not powered.
> @Matt What's your idea?
>