linux-kernel - [PATCH v7 08/13] iio: afe: rescale: fix precision on fractional log scale

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Message-Id: <20210801194000.3646303-9-liambeguin@gmail.com>
Date:   Sun,  1 Aug 2021 15:39:55 -0400
From:   Liam Beguin <liambeguin@...il.com>
To:     liambeguin@...il.com, peda@...ntia.se, jic23@...nel.org,
        lars@...afoo.de, pmeerw@...erw.net
Cc:     linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org,
        devicetree@...r.kernel.org, robh+dt@...nel.org
Subject: [PATCH v7 08/13] iio: afe: rescale: fix precision on fractional log scale

From: Liam Beguin <lvb@...hos.com>

The IIO_VAL_FRACTIONAL_LOG2 scale type doesn't return the expected
scale. Update the case so that the rescaler returns a fractional type
and a more precise scale.

Signed-off-by: Liam Beguin <lvb@...hos.com>
---
 drivers/iio/afe/iio-rescale.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
index abd7ad73d1ce..e37a9766080c 100644
--- a/drivers/iio/afe/iio-rescale.c
+++ b/drivers/iio/afe/iio-rescale.c
@@ -47,12 +47,17 @@ int rescale_process_scale(struct rescale *rescale, int scale_type,
 		*val2 = rescale->denominator;
 		return IIO_VAL_FRACTIONAL;
 	case IIO_VAL_FRACTIONAL_LOG2:
-		tmp = *val * 1000000000LL;
-		do_div(tmp, rescale->denominator);
-		tmp *= rescale->numerator;
-		do_div(tmp, 1000000000LL);
+		if (check_mul_overflow(*val, rescale->numerator, (s32 *)&tmp) ||
+		    check_mul_overflow(rescale->denominator, (1 << *val2), (s32 *)&tmp2)) {
+			tmp = (s64)*val * rescale->numerator;
+			tmp2 = (s64)rescale->denominator * (1 << *val2);
+			factor = gcd(abs(tmp), abs(tmp2));
+			tmp = div_s64(tmp, factor);
+			tmp2 = div_s64(tmp2, factor);
+		}
 		*val = tmp;
-		return scale_type;
+		*val2 = tmp2;
+		return IIO_VAL_FRACTIONAL;
 	case IIO_VAL_INT_PLUS_NANO:
 	case IIO_VAL_INT_PLUS_MICRO:
 		if (scale_type == IIO_VAL_INT_PLUS_NANO)
-- 
2.30.1.489.g328c10930387