linux-kernel - Re: [PATCH v6 09/13] iio: afe: rescale: fix precision on fractional log scale

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Date:   Tue, 27 Jul 2021 20:26:43 -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 v6 09/13] iio: afe: rescale: fix precision on fractional
 log scale

On Fri Jul 23, 2021 at 5:20 PM EDT, Peter Rosin wrote:
> On 2021-07-21 05:06, Liam Beguin wrote:
> > From: Liam Beguin <lvb@...hos.com>
> > 
> > The IIO_VAL_FRACTIONAL_LOG2 scale type doesn't return the expected
> > scale. Update the case so that the rescaler returns a fractional type
> > and a more precise scale.
> > 
> > Signed-off-by: Liam Beguin <lvb@...hos.com>
> > ---
> >  drivers/iio/afe/iio-rescale.c | 9 +++------
> >  1 file changed, 3 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
> > index 35fa3b4e53e0..47cd4a6d9aca 100644
> > --- a/drivers/iio/afe/iio-rescale.c
> > +++ b/drivers/iio/afe/iio-rescale.c
> > @@ -44,12 +44,9 @@ int rescale_process_scale(struct rescale *rescale, int scale_type,
> >  		*val2 = rescale->denominator;
> >  		return IIO_VAL_FRACTIONAL;
> >  	case IIO_VAL_FRACTIONAL_LOG2:
> > -		tmp = *val * 1000000000LL;
> > -		do_div(tmp, rescale->denominator);
> > -		tmp *= rescale->numerator;
> > -		do_div(tmp, 1000000000LL);
> > -		*val = tmp;
> > -		return scale_type;
> > +		*val = rescale->numerator * *val;
> > +		*val2 = rescale->denominator * (1 << *val2);
> > +		return IIO_VAL_FRACTIONAL;
>
> Hi!

Hi Peter,

>
> I do not think this is an uncontested improvement. You have broken the
> case
> where *val2 is "large" before the scale factor is applied.

I was a little reluctant to add this change as I keep increasing the
scope of this series, but since I added tests for all cases, I didn't
want to leave this one out.

Would you rather I drop this patch and the test cases associated to it?

Thanks,
Liam

>
> Cheers,
> Peter
>
> >  	case IIO_VAL_INT_PLUS_NANO:
> >  		tmp = ((s64)*val * 1000000000LL + *val2) * rescale->numerator;
> >  		tmp = div_s64(tmp, rescale->denominator);
> >