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SubjectRe: [PATCH v6 09/13] iio: afe: rescale: fix precision on fractional log scale
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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@xiphos.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@xiphos.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);
> >

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