linux-kernel - Re: [PATCH v4 05/10] iio: afe: rescale: add INT_PLUS

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Date:   Fri, 09 Jul 2021 15:30:23 -0400
From:   "Liam Beguin" <liambeguin@...il.com>
To:     "Peter Rosin" <peda@...ntia.se>, <jic23@...nel.org>,
        <lars@...afoo.de>, <pmeerw@...erw.net>
Cc:     <linux-kernel@...r.kernel.org>, <linux-iio@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <robh+dt@...nel.org>
Subject: Re: [PATCH v4 05/10] iio: afe: rescale: add INT_PLUS_{MICRO,NANO}
 support

On Fri Jul 9, 2021 at 12:29 PM EDT, Peter Rosin wrote:
>
>
> On 2021-07-06 18:09, Liam Beguin wrote:
> > From: Liam Beguin <lvb@...hos.com>
> > 
> > Add IIO_VAL_INT_PLUS_{NANO,MICRO} scaling support.
> > Scale the integer part and the decimal parts individually and keep the
> > original scaling type.
> > 
> > Signed-off-by: Liam Beguin <lvb@...hos.com>
> > ---
> >  drivers/iio/afe/iio-rescale.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
> > index ba3bdcc69b16..1d0e24145d87 100644
> > --- a/drivers/iio/afe/iio-rescale.c
> > +++ b/drivers/iio/afe/iio-rescale.c
> > @@ -89,7 +89,15 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
> >  			do_div(tmp, 1000000000LL);
> >  			*val = tmp;
> >  			return ret;
> > +		case IIO_VAL_INT_PLUS_NANO:
> > +		case IIO_VAL_INT_PLUS_MICRO:
> > +			tmp = (s64)*val * rescale->numerator;
> > +			*val = div_s64(tmp, rescale->denominator);
> > +			tmp = (s64)*val2 * rescale->numerator;
> > +			*val2 = div_s64(tmp, rescale->denominator);
>

Hi Peter,

> Hi!
>
> You are losing precision, and you are not mormalising after the
> calculation.

Can you elaborate a little on what you mean here?

Do you mean that I should make sure that *val2, the PLUS_{NANO,MICRO}
part, doesn't contain an integer part? And if so transfer that part back
to *val?

> I think it's better to not even attempt this given that the results can
> be
> really poor.

Unfortunatelly, I'm kinda stuck with this as some of my ADC use these
types.

Thanks,
Liam

>
> Cheers,
> Peter
>
> > +			return ret;
> >  		default:
> > +			dev_err(&indio_dev->dev, "unsupported type %d\n", ret);
> >  			return -EOPNOTSUPP;
> >  		}
> >  	default:
> >