linux-kernel - Re: [PATCH v4 05/10] iio: afe: rescale: add INT_PLUS

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Message-ID: <4be51a74-9913-291a-9dac-422ac23da3ea@axentia.se>
Date:   Fri, 9 Jul 2021 18:29:15 +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 v4 05/10] iio: afe: rescale: add INT_PLUS_{MICRO,NANO}
 support



On 2021-07-06 18:09, Liam Beguin wrote:
> From: Liam Beguin <lvb@...hos.com>
> 
> Add IIO_VAL_INT_PLUS_{NANO,MICRO} scaling support.
> Scale the integer part and the decimal parts individually and keep the
> original scaling type.
> 
> Signed-off-by: Liam Beguin <lvb@...hos.com>
> ---
>  drivers/iio/afe/iio-rescale.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
> index ba3bdcc69b16..1d0e24145d87 100644
> --- a/drivers/iio/afe/iio-rescale.c
> +++ b/drivers/iio/afe/iio-rescale.c
> @@ -89,7 +89,15 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
>  			do_div(tmp, 1000000000LL);
>  			*val = tmp;
>  			return ret;
> +		case IIO_VAL_INT_PLUS_NANO:
> +		case IIO_VAL_INT_PLUS_MICRO:
> +			tmp = (s64)*val * rescale->numerator;
> +			*val = div_s64(tmp, rescale->denominator);
> +			tmp = (s64)*val2 * rescale->numerator;
> +			*val2 = div_s64(tmp, rescale->denominator);

Hi!

You are losing precision, and you are not mormalising after the calculation.
I think it's better to not even attempt this given that the results can be
really poor.

Cheers,
Peter

> +			return ret;
>  		default:
> +			dev_err(&indio_dev->dev, "unsupported type %d\n", ret);
>  			return -EOPNOTSUPP;
>  		}
>  	default:
>