Re: [PATCH] iio: adc: Add Texas Instruments ADC081C021/027 support

[Thierry Reding <thierry.reding@avionic-design.de>]

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On Mon, Nov 19, 2012 at 10:33:48AM +0100, Lars-Peter Clausen wrote:
> On 11/19/2012 09:21 AM, Thierry Reding wrote:
> > Add support for reading conversion results from the ADC and provide them
> > through a single IIO channel. A proper scaling factor is also exported
> > based on the reference voltage provided by a regulator.
> > 
> > Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
> > ---
> [...]
> > +static int adc081c_read_raw(struct iio_dev *iio,
> > +			    struct iio_chan_spec const *channel, int *value,
> > +			    int *micro, long mask)
> > +{
> > +	struct adc081c *adc = iio_priv(iio);
> > +	int err, scale;
> > +
> > +	switch (mask) {
> > +	case IIO_CHAN_INFO_RAW:
> > +		err = i2c_smbus_read_word_swapped(adc->i2c, REG_CONV_RES);
> > +		if (err < 0)
> > +			return err;
> > +
> > +		*value = (err >> 4) & 0xff;
> > +		return IIO_VAL_INT;
> > +
> > +	case IIO_CHAN_INFO_SCALE:
> > +		err = regulator_get_voltage(adc->ref);
> > +		if (err < 0)
> > +			return err;
> > +
> > +		scale = err / 255;
> 
> Shouldn't this be 256?

Well, the maximum value that the conversion register can contain is 255,
so if we divide by 256 we'll always be slightly off, right? Or maybe I
misunderstand what the scale factor is supposed to do. From reading the
documentation it seems like user-space is supposed to multiply the raw
value by the scale to obtain the real voltage.

> > +
> > +		*value = scale / 1000000;
> > +		*micro = scale % 1000000;
> 
> scale for voltages is in microvolt, so I think it this is off by a factor of
> 1000.

If scale is supposed to be in microvolts, then this is off by a factor
of 1000000 since regulator_get_voltage() returns the voltage in
microvolts as well.

> For ADCs it often makes sense to use IIO_VAL_FRACTIONAL_LOG2 with the val
> being set to the reference voltage (in mV) and val2 being set to the number
> of bits.
> 
> E.g in your case
> 
> 		*val = err / 1000;
> 		*val2 = 8;

I'm confused, IIO_VAL_FRACTIONAL_LOG2 is supposed to return mV, but
IIO_VAL_INT_PLUS_MICRO will return uV?

And setting val2 to 8 will cause the scaling factor to be 256, and not
255, so the converted value will actually be off (as I mentioned above).

> > +
> > +		return IIO_VAL_INT_PLUS_MICRO;
[...]
> > +	adc->channel.type = IIO_VOLTAGE;
> > +	adc->channel.info_mask = IIO_CHAN_INFO_SCALE_SHARED_BIT |
> > +				 IIO_CHAN_INFO_RAW_SEPARATE_BIT;
> 
> nitpick: Since it is the same for each driver I'd make the channel static const.

Okay, that makes sense.

Thierry

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