linux-kernel - Re: [PATCH v5 05/10] iio: afe: rescale: add INT_PLUS

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Message-Id: <CCUT1ZDDWS1J.3CGKX5J1MNFOX@shaak>
Date:   Fri, 16 Jul 2021 15:18:33 -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 v5 05/10] iio: afe: rescale: add INT_PLUS_{MICRO,NANO}
 support

On Thu Jul 15, 2021 at 5:48 AM EDT, Peter Rosin wrote:
>
> On 2021-07-15 05:12, Liam Beguin wrote:
> > From: Liam Beguin <lvb@...hos.com>
> > 
> > Some ADCs use IIO_VAL_INT_PLUS_{NANO,MICRO} scale types.
> > Add support for these to allow using the iio-rescaler with them.
> > 
> > Signed-off-by: Liam Beguin <lvb@...hos.com>
> > ---
> >  drivers/iio/afe/iio-rescale.c | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> > 
> > diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
> > index 4c3cfd4d5181..a2b220b5ba86 100644
> > --- a/drivers/iio/afe/iio-rescale.c
> > +++ b/drivers/iio/afe/iio-rescale.c
> > @@ -92,7 +92,22 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
> >  			do_div(tmp, 1000000000LL);
> >  			*val = tmp;
> >  			return ret;
> > +		case IIO_VAL_INT_PLUS_NANO:
> > +			tmp = ((s64)*val * 1000000000LL + *val2) * rescale->numerator;
> > +			do_div(tmp, rescale->denominator);
> > +
> > +			*val = div_s64(tmp, 1000000000LL);
> > +			*val2 = tmp - *val * 1000000000LL;
> > +			return ret;
>
> This is too simplistic and prone to overflow. We need something like
> this
> (untested)
>
> tmp = (s64)*val * rescale->numerator;
> rem = do_div(tmp, rescale->denominator);
> *val = tmp;
> tmp = ((s64)rem * 1000000000LL + (s64)*val2) * rescale->numerator;
> do_div(tmp, rescale->denominator);
> *val2 = tmp;
>
> Still not very safe with numerator and denominator both "large", but
> much
> better. And then we need normalizing the fraction part after the above,
> of
> course.
>

Understood, I'll test that.

> And, of course, I'm not sure what *val == -1 and *val2 == 500000000
> really
> means. Is that -1.5 or -0.5? The above may very well need adjusting for
> negative values...
>

I would've assumed the correct answer is -1 + 500000000e-9 = -0.5
but adding a test case to iio-test-format.c seems to return -1.5...

I believe that's a bug but we can work around if for now by moving the
integer part of *val2 to *val.

Liam

> Cheers,
> Peter
>
> > +		case IIO_VAL_INT_PLUS_MICRO:
> > +			tmp = ((s64)*val * 1000000LL + *val2) * rescale->numerator;
> > +			do_div(tmp, rescale->denominator);
> > +
> > +			*val = div_s64(tmp, 1000000LL);
> > +			*val2 = tmp - *val * 1000000LL;
> > +			return ret;
> >  		default:
> > +			dev_err(&indio_dev->dev, "unsupported type %d\n", ret);
> >  			return -EOPNOTSUPP;
> >  		}
> >  	default:
> >