Re: [LINUX PATCH v3] iio: core: Fix IIO_VAL_FRACTIONAL calculation for negative values

[Jonathan Cameron <jic23@kernel.org>]

On Mon,  5 Oct 2020 08:05:16 -0700
Anand Ashok Dumbre <anand.ashok.dumbre@xilinx.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@xilinx.com>
> Reported-by: kernel test robot <lkp@intel.com> #error: uninitialized symbol tmp
> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
As this doesn't have a fixes tag etc and from v2 discussion was only hit
with a new driver, I'm not currently taking this a a fix.
If people want me to rush it in / backport to stable then let me know

Applied to the togreg branch of iio.git and pushed out as testing for
the autobuilders to poke at it.

Thanks,

Jonathan

> ---
> 
> Changes in v3:
> 	Fixed a bug caught by kernel test robot and used correct variable
> 
> ---
>  drivers/iio/industrialio-core.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
> index cdcd16f1..ffd5176 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));
> +		tmp0 = (int)div_s64_rem(tmp2, 1000000000, &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);