Hi Jonathan,

I encountered this when I was developing a new driver.
If you look at the function where this is used, all other IIO_VAL_MICRO and NANO
have this fix added at some point.

Thanks,
Anand

-----Original Message-----
From: Jonathan Cameron <jic23@kernel.org>
Sent: Saturday, August 29, 2020 4:19 PM
To: Anand Ashok Dumbre <redacted>
Cc: knaack.h@gmx.de; lars@metafoo.de; pmeerw@pmeerw.net; Michal
Simek [off-list ref]; git [off-list ref]; linux-
iio@vger.kernel.org; linux-arm-kernel@lists.infradead.org; linux-
kernel@vger.kernel.org; Anand Ashok Dumbre [off-list ref]
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 [off-list ref] wrote:

quoted

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 <redacted>

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

quoted

---
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,

quoted

 		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));

quoted

 	case IIO_VAL_FRACTIONAL_LOG2:
 		tmp = shift_right((s64)vals[0] * 1000000000LL, vals[1]);
 		tmp0 = (int)div_s64_rem(tmp, 1000000000LL, &tmp1);

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