[PATCH v5 1/6] iio: consumers: Fix handling of negative channel scale in iio_convert_raw_to_processed()

[Hans de Goede <hansg@kernel.org>]

There is an issue with the handling of negative channel scales
in iio_convert_raw_to_processed_unlocked() when the channel-scale
is of the IIO_VAL_INT_PLUS_[MICRO|NANO] type:

Things work for channel-scale values > -1.0 and < 0.0 because of
the use of signed values in:

	*processed += div_s64(raw64 * (s64)scale_val2 * scale, 1000000LL);

Things will break however for scale values < -1.0. Lets for example say
that raw = 2, (caller-provided)scale = 10 and (channel)scale_val = -1.5.

The result should then be 2 * 10 * -1.5 = -30.

channel-scale = -1.5 means scale_val = -1 and scale_val2 = 500000,
now lets see what gets stored in processed:

1. *processed = raw64 * scale_val * scale;
2. *processed += raw64 * scale_val2 * scale / 1000000LL;

1. Sets processed to 2 * -1 * 10 = -20
2. Adds 2 * 500000 * 10 / 1000000 = 10 to processed

And the end result is processed = -20 + 10 = -10, which is not correct.

Fix this by always using the abs value of both scale_val and scale_val2
and if either is negative multiply the end-result by -1.

Note there seems to be an unwritten rule about negative
IIO_VAL_INT_PLUS_[MICRO|NANO] values that:

i.   values > -1.0 and < 0.0 are written as val=0 val2=-xxx
ii.  values <= -1.0 are written as val=-xxx val2=xxx

But iio_format_value() will also correctly display a third option:

iii. values <= -1.0 written as val=-xxx val2=-xxx

Since iio_format_value() uses abs(val) when val2 < 0.

This fix also makes iio_convert_raw_to_processed() properly handle
channel-scales using this third option.

Fixes: 48e44ce0f881 ("iio:inkern: Add function to read the processed value")
Cc: Matteo Martelli <matteomartelli3@gmail.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Hans de Goede <hansg@kernel.org>
---
Changes in v5:
- Do not put case foo: val = x; break; statements on a single line

Changes in v4:
- New patch in v4 of this patch-set
---
 drivers/iio/inkern.c | 28 ++++++++++++++--------------
 1 file changed, 14 insertions(+), 14 deletions(-)

diff --git a/drivers/iio/inkern.c b/drivers/iio/inkern.c
index c174ebb7d5e6..d36a80a7b8a9 100644
--- a/drivers/iio/inkern.c
+++ b/drivers/iio/inkern.c
@@ -11,6 +11,7 @@
 #include <linux/mutex.h>
 #include <linux/property.h>
 #include <linux/slab.h>
+#include <linux/units.h>
 
 #include <linux/iio/iio.h>
 #include <linux/iio/iio-opaque.h>
@@ -604,7 +605,7 @@ static int iio_convert_raw_to_processed_unlocked(struct iio_channel *chan,
 {
 	int scale_type, scale_val, scale_val2;
 	int offset_type, offset_val, offset_val2;
-	s64 raw64 = raw;
+	s64 denominator, raw64 = raw;
 
 	offset_type = iio_channel_read(chan, &offset_val, &offset_val2,
 				       IIO_CHAN_INFO_OFFSET);
@@ -648,20 +649,19 @@ static int iio_convert_raw_to_processed_unlocked(struct iio_channel *chan,
 		*processed = raw64 * scale_val * scale;
 		break;
 	case IIO_VAL_INT_PLUS_MICRO:
-		if (scale_val2 < 0)
-			*processed = -raw64 * scale_val * scale;
-		else
-			*processed = raw64 * scale_val * scale;
-		*processed += div_s64(raw64 * (s64)scale_val2 * scale,
-				      1000000LL);
-		break;
 	case IIO_VAL_INT_PLUS_NANO:
-		if (scale_val2 < 0)
-			*processed = -raw64 * scale_val * scale;
-		else
-			*processed = raw64 * scale_val * scale;
-		*processed += div_s64(raw64 * (s64)scale_val2 * scale,
-				      1000000000LL);
+		switch (scale_type) {
+		case IIO_VAL_INT_PLUS_MICRO:
+			denominator = MICRO;
+			break;
+		case IIO_VAL_INT_PLUS_NANO:
+			denominator = NANO;
+			break;
+		}
+		*processed = raw64 * scale * abs(scale_val);
+		*processed += div_s64(raw64 * scale * abs(scale_val2), denominator);
+		if (scale_val < 0 || scale_val2 < 0)
+			*processed *= -1;
 		break;
 	case IIO_VAL_FRACTIONAL:
 		*processed = div_s64(raw64 * (s64)scale_val * scale,
-- 
2.51.0