Re: [2/3] iio: adc: ina2xx: Adhere to documented ABI, use Ohm instead of uOhm

[Maciej Purski <m.purski@samsung.com>]

On 11/06/2017 11:21 AM, Stefan Brüns wrote:
> On Thursday, November 2, 2017 10:04:01 AM CET Maciej Purski wrote:
>> On 10/14/2017 08:27 PM, Stefan Bruens wrote:
>>> On Montag, 9. Oktober 2017 11:29:43 CEST Maciej Purski wrote:
>>>> On 10/01/2017 09:48 PM, Stefan Brüns wrote:
>>>>> According to the ABI documentation, the shunt resistor value should be
>>>>> specificied in Ohm. As this is also used/documented for the MAX9611,
>>>>> use the same for the INA2xx driver.
>>>>>
>>>>> This poses an ABI break for anyone actually altering the shunt value
>>>>> through the sysfs interface, it does not alter the default value nor
>>>>> a value set from the devicetree.
>>>>>
>>>>> Minor change: Fix comment, 1mA is 10^-3A.
>>>>
>>>> I have just a minor issue. There could be an inconsistency with units as
>>>> in
>>>> my patch I make current_lsb adjustable and I need it to be in uA (it used
>>>> to be hardcoded as 1 mA so to achieve better precision we need smaller
>>>> units). So in order to keep calibration register properly scaled, I
>>>> convert
>>>> uOhms to mOhms on each set_calibration(). So if both my changes and your
>>>> changes were applied, on each shunt_resistore_store we would be
>>>> performing
>>>> multiplication by 10^6 and then in set_calibration() division by 10^3
>>>> which
>>>> seems odd to me.
>>>>
>>>> I guess we could keep it as shunt_resistor_ohms instead of
>>>> shunt_resistor_uohm. We could avoid performing division on each
>>>> shunt_resistor_show() and perform multiplication by 10^3 only once in
>>>> set_calibration() on each
>>>> shunt_resistore_store(). We could then change the default value and
>>>> perform
>>>> division only on probing, when reading the shunt_resistance from device
>>>> tree.
>>>>
>>>> There are many other options. It's not a major issue so maybe we could
>>>> leave it as it is or you could suggest some changes in my patch.
>>>
>>> Sorry it took me so long to answer ...
>>>
>>> The current fixed current_lsb of 1mA is indeed a bad choice for everything
>>> but a shunt resistor value of 10mOhm, as it truncates the current value.
>>> So what is a *good* choice?
>>>
>>> One important point is the current register is merely more than a
>>> convenience register. At least for the INA219/220, it provides nothing
>>> not achievable in software, and for the INA226 family it only has added
>>> value if the current is varying faster than the readout frequency and the
>>> averaging is used.
>>>
>>> The precision of the current register is limited by the precision of the
>>> shunt voltage register, and may be reduced by the applied
>>> scaling/calibration factor.
>>>
>>> The precision of the shunt voltage register is fixed at 10uV (INA219)
>>> resp.
>>> 2.5uV (INA226). Changing conversion time (both) and PGA (219) affects the
>>> noise and offset, but the lsb value is still fixed.
>>>
>>> If one wants to carry over the shunt voltage register precision into the
>>> current register, its important no (or hardly any) truncation happens. The
>>> terms therefor are given in the manual, formulas 8.5.1 (4) resp 7.5.1 (3):
>>>
>>> INA219: current = shunt_voltage * cal_register / 4096
>>> INA226: current = shunt_voltage * cal_register / 2048
>>>
>>> So any cal value smaller than 4096 (2048) will introduce truncation
>>> errors,
>>> larger values may introduce overflows, if the full input range is used.
>>> Now, would it not be wise to always use 4096 (2048) for the calibration
>>> value?
>>>
>>> The raw values from the IIO subsystem are meaningless without their
>>> accompanying scale factor. Instead of changing the calibration value, why
>>> not just change the reported scale factor?
>>>
>>> More opinions are very welcome.
>>>
>>> Kind regards,
>>>
>>> Stefan
>>
>> Thanks for the reply.
>>
>> I agree that cal_register set to 4096 (2048) allows us to eliminate
>> truncaction error. However according to your suggestion, if we made cal_reg
>> a fixed value, then current_lsb and r_shunt should be also a fixed value,
>> as they are related according to formula 8.5 (1)
>>
>> cal_register = 0.00512 / (current_lsb * r_shunt)
> 
> A fixed cal_register only means the current_lsb is implied by the selected
> shunt resistor value.
> 
> If you insert 2048 into the equation above, you get:
> 
> current_lsb = 2.5 * 1e-6 * r_shunt,
> 
> and using Ohms law to replace r_shunt, thats exactly the resolution of the
> shunt_voltage register as specified in the datasheet. The higher the shunt
> resistor value, the smaller the current_lsb.
>   
>> Therefore, changing the scale value wouldn't affect the calib_reg value, so
>> it wouldn't give the user any information on the actual current_lsb of the
>> device. The real value is calculated like this by the user:
>>
>> processed_value = raw_value * scale
>>
>> I think that even after changing the scale value processed_value is expected
>> to be approximately the same.
> 
> A fixed cal_register means you change the current_lsb by changing the shunt
> resistor. This exposes the full ADC resolution.
>   
> The current_lsb *is* the scale value.
> 
> Kind regards,
> 
> Stefan
> 

Thanks for your explanation. I can do this the way you suggest, so the only 
change with the original driver would be to make current_lsb (which is a scale 
value) follow changes of shunt_resistance value from userspace.

But before I'd like to ask Jonathan for opinion on that.

Kind regards,

Maciej