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

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Date:   Fri, 23 Jul 2021 23:20:09 +0200
From:   Peter Rosin <peda@...ntia.se>
To:     Liam Beguin <liambeguin@...il.com>, 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 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!

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);
>