linux-kernel - [PATCH v9 09/14] iio: afe: rescale: fix accuracy for small fractional scales

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Message-Id: <20211115034334.1713050-10-liambeguin@gmail.com>
Date:   Sun, 14 Nov 2021 22:43:29 -0500
From:   Liam Beguin <liambeguin@...il.com>
To:     liambeguin@...il.com, peda@...ntia.se, jic23@...nel.org,
        lars@...afoo.de
Cc:     linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org,
        devicetree@...r.kernel.org, robh+dt@...nel.org
Subject: [PATCH v9 09/14] iio: afe: rescale: fix accuracy for small fractional scales

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

The approximation caused by integer divisions can be costly on smaller
scale values since the decimal part is significant compared to the
integer part. Switch to an IIO_VAL_INT_PLUS_NANO scale type in such
cases to maintain accuracy.

Signed-off-by: Liam Beguin <lvb@...hos.com>
---
 drivers/iio/afe/iio-rescale.c | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
index 17036130d364..8a2f1c0ca5a3 100644
--- a/drivers/iio/afe/iio-rescale.c
+++ b/drivers/iio/afe/iio-rescale.c
@@ -22,7 +22,7 @@ int rescale_process_scale(struct rescale *rescale, int scale_type,
 			  int *val, int *val2)
 {
 	s64 tmp;
-	s32 rem;
+	s32 rem, rem2;
 	u32 mult;
 	u32 neg;
 
@@ -38,9 +38,26 @@ int rescale_process_scale(struct rescale *rescale, int scale_type,
 		tmp = (s64)*val * 1000000000LL;
 		tmp = div_s64(tmp, rescale->denominator);
 		tmp *= rescale->numerator;
-		tmp = div_s64(tmp, 1000000000LL);
+
+		tmp = div_s64_rem(tmp, 1000000000LL, &rem);
 		*val = tmp;
-		return scale_type;
+
+		if (!rem)
+			return scale_type;
+
+		if (scale_type == IIO_VAL_FRACTIONAL)
+			tmp = *val2;
+		else
+			tmp = 1 << *val2;
+
+		rem2 = *val % (int)tmp;
+		*val = *val / (int)tmp;
+
+		*val2 = rem / (int)tmp;
+		if (rem2)
+			*val2 += div_s64((s64)rem2 * 1000000000LL, tmp);
+
+		return IIO_VAL_INT_PLUS_NANO;
 	case IIO_VAL_INT_PLUS_NANO:
 	case IIO_VAL_INT_PLUS_MICRO:
 		if (scale_type == IIO_VAL_INT_PLUS_NANO)
-- 
2.32.0.452.g940fe202adcb