linux-kernel - RE: [PATCH v5] iio: adc: add new lp8788 adc driver

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Date:	Fri, 14 Sep 2012 00:33:27 +0000
From:	"Kim, Milo" <Milo.Kim@...com>
To:	Lars-Peter Clausen <lars@...afoo.de>
CC:	Jonathan Cameron <jic23@...nel.org>,
	Jonathan Cameron <jic23@....ac.uk>,
	"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v5] iio: adc: add new lp8788 adc driver

> Hi,
> 
> One issue and a couple of nitpicks inline.

I really appreciate it.
Please see my questions below.

> > +	switch (mask) {
> > +	case IIO_CHAN_INFO_RAW:
> > +		ret = lp8788_get_adc_result(adc, id, val) ? -EIO :
> IIO_VAL_INT;
> > +		break;
> > +	case IIO_CHAN_INFO_SCALE:
> > +		*val = (lp8788_scale[id] / 1000) * 1000;
> > +		*val2 = (lp8788_scale[id] % 1000) * 1000000;
> 
> This looks suspicious. E.g. if the entry in your table has the value
> "1234"
> you'd end up with a scale factor of 1000.234000000. Which looks wrong
> for
> two reasons, given that you return INT_PLUS_MICRO val2 should never
> exceed 6
> decimal digits and secondly val is multiplied by a factor of thousand.
> 
> Can you tell us in what unit the values in lp8788_scale are, that would
> make
> review easier.

The LP8788 has 13 ADC input selection.

ADC selection:
Battery voltage, general ADC1 and so on.

ADC result:
Result = MAX_VALUE * (raw + 0.5) / 4095  except ADC is the charger voltage
If the ADC input is the charger voltage,
Result = MAX_VALUE * (raw + 0.5) / (4095 * 0.48)

The raw value is from the registers.
It has the range between 0 to 4095. (12bits)

MAX_VALUE is constant for each selection.
For the battery voltage, there are three ADC inputs. 5.0/5.5/6.0V
Battery voltage for Max 5.0V = 5.0
Battery voltage for Max 5.5V = 5.5
Battery voltage for Max 6.0V = 6.0
Charger = 6.0
ADC1 = 2.5 

I'm afraid I still misunderstand how IIO ADC works.
Could you me some guide how to setup the scale in the driver?

Thank you.

Best Regards,
Milo
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