linux-kernel - Re: [PATCH 1/1] iio: core: Fix fractional format generation

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Date:   Wed, 21 Aug 2019 22:09:21 +0200
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Alexander Stein <alexander.stein@...tec-electronic.com>,
        Jonathan Cameron <jic23@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>
Cc:     linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] iio: core: Fix fractional format generation

On 8/21/19 5:50 PM, Alexander Stein wrote:
> In case the result is -0.3252 tmp0 is 0 after the div_s64_rem, so tmp0 is
> non-negative which results in an output of 0.3252.
> Fix this by explicitly handling the negative sign ourselves.

Hi,

Thanks for you patch. Some comments inline.

> 
> Signed-off-by: Alexander Stein <alexander.stein@...tec-electronic.com>
> ---
>  drivers/iio/industrialio-core.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
> index 245b5844028d..18350c1959ae 100644
> --- a/drivers/iio/industrialio-core.c
> +++ b/drivers/iio/industrialio-core.c
> @@ -568,6 +568,7 @@ static ssize_t __iio_format_value(char *buf, size_t len, unsigned int type,
>  {
>  	unsigned long long tmp;
>  	int tmp0, tmp1;
> +	const char *sign = vals[0] < 0 ? "-" : "";
>  	bool scale_db = false;
>  
>  	switch (type) {
> @@ -593,11 +594,11 @@ static ssize_t __iio_format_value(char *buf, size_t len, unsigned int type,
>  		tmp = div_s64((s64)vals[0] * 1000000000LL, vals[1]);
>  		tmp1 = vals[1];
>  		tmp0 = (int)div_s64_rem(tmp, 1000000000, &tmp1);
> -		return snprintf(buf, len, "%d.%09u", tmp0, abs(tmp1));
> +		return snprintf(buf, len, "%s%u.%09u", sign, abs(tmp0), abs(tmp1));

I think this breaks the case where vals[1] is negative, but vals[0] is
positive. Maybe we can use a similar approach as for
IIO_VAL_INT_PLUS_NANO. Maybe even put this into a small helper function.

>  	case IIO_VAL_FRACTIONAL_LOG2:
>  		tmp = shift_right((s64)vals[0] * 1000000000LL, vals[1]);
>  		tmp0 = (int)div_s64_rem(tmp, 1000000000LL, &tmp1);
> -		return snprintf(buf, len, "%d.%09u", tmp0, abs(tmp1));
> +		return snprintf(buf, len, "%s%u.%09u", sign, abs(tmp0), abs(tmp1));
>  	case IIO_VAL_INT_MULTIPLE:
>  	{
>  		int i;
>