[PATCH v2] iio: hid-sensors: fix fractional part for unit exponents -7 and -8

[PATCH v2] iio: hid-sensors: fix fractional part for unit exponents -7 and -8

[Srinivas Pandruvada]

split_micro_fraction() scales the remainder to micro units by
multiplying it by 10^(6 - exp). The unit exponent of a HID field goes
down to -8, so exp can be 7 or 8 here. Then the argument of int_pow()
is negative, which the unsigned parameter turns into a huge power of
ten: 0 modulo 2^64. As a result, the fractional part of a value read
from the hysteresis attributes is always reported as 0.

Divide by 10^(exp - 6) in these two cases. This matches the reverse
conversion in convert_to_vtf_format().

Fixes: 73c6768b710a ("iio: hid-sensors: Common attribute and trigger")
Cc: stable@vger.kernel.org
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
---
v2:
- Fix __udivdi3 link error on 32-bit by using a 32-bit divisor
- Address Andy's comment:
Keep no % divisor in a temporary variable next to no / divisor

 drivers/iio/common/hid-sensors/hid-sensor-attributes.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/iio/common/hid-sensors/hid-sensor-attributes.c b/drivers/iio/common/hid-sensors/hid-sensor-attributes.c
index 9e7c26e37880..d44091faf122 100644
--- a/drivers/iio/common/hid-sensors/hid-sensor-attributes.c
+++ b/drivers/iio/common/hid-sensors/hid-sensor-attributes.c
@@ -96,9 +96,14 @@ static void simple_div(int dividend, int divisor, int *whole, int *micro_frac)
 static void split_micro_fraction(unsigned int no, int exp, int *val1, int *val2)
 {
 	int divisor = int_pow(10, exp);
+	unsigned int rem;
 
 	*val1 = no / divisor;
-	*val2 = no % divisor * int_pow(10, 6 - exp);
+	rem = no % divisor;
+	if (exp > 6)
+		*val2 = rem / (unsigned int)int_pow(10, exp - 6);
+	else
+		*val2 = rem * int_pow(10, 6 - exp);
 }
 
 /*
-- 
2.56.0