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SubjectRe: [PATCH v6 09/13] iio: afe: rescale: fix precision on fractional log scale
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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!

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

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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