linux-kernel - Re: [PATCH v2] iio: core: Fix IIO_VAL_FRACTIONAL calculation for negative values

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Date:   Sat, 29 Aug 2020 16:19:00 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     Anand Ashok Dumbre <anand.ashok.dumbre@...inx.com>
Cc:     knaack.h@....de, lars@...afoo.de, pmeerw@...erw.net,
        michal.simek@...inx.com, git@...inx.com, linux-iio@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        anandash@...inx.com
Subject: Re: [PATCH v2] iio: core: Fix IIO_VAL_FRACTIONAL calculation for
 negative values

On Wed, 26 Aug 2020 11:14:36 -0700
Anand Ashok Dumbre <anand.ashok.dumbre@...inx.com> wrote:

> Fixes IIO_VAL_FRACTIONAL for case when the result is negative and
> exponent is 0.
> 
> example: if the result is -0.75, tmp0 will be 0 and tmp1 = 75
> This causes the output to lose sign because of %d in snprintf
> which works for tmp0 <= -1.
> 
> Signed-off-by: Anand Ashok Dumbre <anand.ashok.dumbre@...inx.com>

Looks good.  Just one last thing.

Is this actually hit in an existing driver?  I'm just wondering
how far back we need to push it in stable etc.

Thanks,

Jonathan

> ---
> changes since v1:
> 	Changed -%d to -0 to make the fix clearer.
> 	Removed the email footer.
> 	Updated the commit description with an example
> --
>  drivers/iio/industrialio-core.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
> index cdcd16f1..a239fa2 100644
> --- a/drivers/iio/industrialio-core.c
> +++ b/drivers/iio/industrialio-core.c
> @@ -592,6 +592,7 @@ static ssize_t __iio_format_value(char *buf, size_t len, unsigned int type,
>  {
>  	unsigned long long tmp;
>  	int tmp0, tmp1;
> +	s64 tmp2;
>  	bool scale_db = false;
>  
>  	switch (type) {
> @@ -614,10 +615,13 @@ static ssize_t __iio_format_value(char *buf, size_t len, unsigned int type,
>  		else
>  			return scnprintf(buf, len, "%d.%09u", vals[0], vals[1]);
>  	case IIO_VAL_FRACTIONAL:
> -		tmp = div_s64((s64)vals[0] * 1000000000LL, vals[1]);
> +		tmp2 = div_s64((s64)vals[0] * 1000000000LL, vals[1]);
>  		tmp1 = vals[1];
>  		tmp0 = (int)div_s64_rem(tmp, 1000000000, &tmp1);
> -		return scnprintf(buf, len, "%d.%09u", tmp0, abs(tmp1));
> +		if ((tmp2 < 0) && (tmp0 == 0))
> +			return snprintf(buf, len, "-0.%09u", abs(tmp1));
> +		else
> +			return snprintf(buf, len, "%d.%09u", tmp0, abs(tmp1));
>  	case IIO_VAL_FRACTIONAL_LOG2:
>  		tmp = shift_right((s64)vals[0] * 1000000000LL, vals[1]);
>  		tmp0 = (int)div_s64_rem(tmp, 1000000000LL, &tmp1);