linux-kernel - Re: [PATCH v5 05/10] iio: afe: rescale: add INT_PLUS

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Date:   Thu, 15 Jul 2021 11:48:31 +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 v5 05/10] iio: afe: rescale: add INT_PLUS_{MICRO,NANO}
 support


On 2021-07-15 05:12, Liam Beguin wrote:
> From: Liam Beguin <lvb@...hos.com>
> 
> Some ADCs use IIO_VAL_INT_PLUS_{NANO,MICRO} scale types.
> Add support for these to allow using the iio-rescaler with them.
> 
> Signed-off-by: Liam Beguin <lvb@...hos.com>
> ---
>  drivers/iio/afe/iio-rescale.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
> index 4c3cfd4d5181..a2b220b5ba86 100644
> --- a/drivers/iio/afe/iio-rescale.c
> +++ b/drivers/iio/afe/iio-rescale.c
> @@ -92,7 +92,22 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
>  			do_div(tmp, 1000000000LL);
>  			*val = tmp;
>  			return ret;
> +		case IIO_VAL_INT_PLUS_NANO:
> +			tmp = ((s64)*val * 1000000000LL + *val2) * rescale->numerator;
> +			do_div(tmp, rescale->denominator);
> +
> +			*val = div_s64(tmp, 1000000000LL);
> +			*val2 = tmp - *val * 1000000000LL;
> +			return ret;

This is too simplistic and prone to overflow. We need something like this
(untested)

	tmp = (s64)*val * rescale->numerator;
	rem = do_div(tmp, rescale->denominator);
	*val = tmp;
	tmp = ((s64)rem * 1000000000LL + (s64)*val2) * rescale->numerator;
	do_div(tmp, rescale->denominator);
	*val2 = tmp;

Still not very safe with numerator and denominator both "large", but much
better. And then we need normalizing the fraction part after the above, of
course.

And, of course, I'm not sure what *val == -1 and *val2 == 500000000 really
means. Is that -1.5 or -0.5? The above may very well need adjusting for
negative values...

Cheers,
Peter

> +		case IIO_VAL_INT_PLUS_MICRO:
> +			tmp = ((s64)*val * 1000000LL + *val2) * rescale->numerator;
> +			do_div(tmp, rescale->denominator);
> +
> +			*val = div_s64(tmp, 1000000LL);
> +			*val2 = tmp - *val * 1000000LL;
> +			return ret;
>  		default:
> +			dev_err(&indio_dev->dev, "unsupported type %d\n", ret);
>  			return -EOPNOTSUPP;
>  		}
>  	default:
>