Re: [PATCH v3 4/5] iio: mxs-lradc: add scale_available file to channels

[Hector Palacios]

Hello Marek,

On 07/23/2013 12:36 AM, Marek Vasut wrote:
> Dear Hector Palacios,
>
> > Adds in_voltageX_scale_available file for every channel to read
> > the different available scales.
> > There are two scales per channel:
> >   [0] = divider_by_two disabled (default)
> >   [1] = divider_by_two enabled
> > The scale is a struct made of integer and nano parts to build
> > a long decimal number.
> >
> > Signed-off-by: Hector Palacios <hector.palacios@xxxxxxxx>
> > ---
>
> [...]
>
> > @@ -1008,6 +1098,26 @@ static int mxs_lradc_probe(struct platform_device
> > *pdev) if (ret)
> >   		goto err_trig;
> >
> > +	/* Populate available ADC input ranges */
> > +	for (i = 0; i < LRADC_MAX_TOTAL_CHANS; i++) {
> > +		for (s = 0; s < ARRAY_SIZE(lradc->scale_avail[i]); s++) {
> > +			/*
> > +			 * [s=0] = optional divider by two disabled (default)
> > +			 * [s=1] = optional divider by two enabled
> > +			 *
> > +			 * The scale is calculated by doing:
> > +			 *   Vref >> (realbits - s)
> > +			 * which multiplies by two on the second component
> > +			 * of the array.
> > +			 */
> > +			scale_uv = ((u64)lradc->vref_mv[i] * 100000000) >>
> > +				   (iio->channels[i].scan_type.realbits - s);
>
> Thinking about this, this is basically
>
>              vref_mv[CHANNEL] * 100 000 000
> scale_uv = --------------------------------
>                        2^(12 - s)
>
> Where s can be either 0 or 1.
>
> Why do you multiply it by 100000000 I don't quite understand, but maybe it's
> fully obvious. It should be documented though.

I copied Michael Hennerich in CC who was the author of this math in
 drivers/iio/adc/ad7793.c
 drivers/staging/iio/adc/ad7192.c
which I copied. Maybe he can explain. It looks like it is just a question of having a 
intermediate large enough number to later do the do_div operation, rather than having 
a meaningful figure.

> > +			lradc->scale_avail[i][s].nano =
> > +					do_div(scale_uv, 100000000) * 10;
>
> Are we not loosing precission here?

Yes, but the do_div is modifying the scale_uv parameter with the integer part so I 
guess it might be somewhat justified.
As I said before, I tried to do my own math without success so I just copied what was 
in other drivers.

> > +			lradc->scale_avail[i][s].integer = scale_uv;
> > +		}
> > +	}
> > +
> >   	/* Configure the hardware. */
> >   	mxs_lradc_hw_init(lradc);
>
> Best regards,
> Marek Vasut


Best regards,
--
Hector Palacios
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