Re: [PATCH v2] hwmon: (ntc_thermistor): try reading processed

[Chris Lesiak <chris.lesiak@licor.com>]

Linus Walleij <linus.walleij@linaro.org>wrote:
> I have to do this change because my ADC driver only provides processed
> channels (drivers/iio/adc/ab8500-gpadc.c). It provides raw values and
> it provides processed values but no scale. That means your code will
> not work, sadly. It will result in the raw value being used without scaling.

> The reason that the ADC cannot provide scaling is that the scale is not
> linear and based on calibration. IIO scaling is only linear.

I haven't been able to find detailed documentation on the ab8500-gpadc,
so I have a couple of questions / comments:

1. The driver appears to support temperature output directly.  Why do
you need ntc_thermistor?

2. I don't understand how the ab8500_gpadc_read_raw output of processed
data could possibly be correct.

        if (mask == IIO_CHAN_INFO_PROCESSED) {
                processed = ab8500_gpadc_ad_to_voltage(gpadc, ch->id, raw_val);
                if (processed < 0)
                        return processed;

                /* Return millivolt or milliamps or millicentigrades */
                *val = processed * 1000;
                return IIO_VAL_INT;
        }

Note that both processed and *val are both of type int.

If *val really does end up with milliVolt units, then processed must
have had Volt units.  And you only have single Volt resolution.

Either you are working with a lot higher voltages than I usually see,
or something must be wrong.