[PATCH v5 4/6] iio: Improve iio_read_channel_processed_scale() precision

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

Before this change iio_read_channel_processed_scale() always assumes that
channels which advertise IIO_CHAN_INFO_PROCESSED capability return
IIO_VAL_INT on success.

Ignoring any fractional values from drivers which return
IIO_VAL_INT_PLUS_MICRO / IIO_VAL_INT_PLUS_NANO. These fractional values
might become non fractional after scaling so these should be taken into
account for better precision.

Use the new iio_multiply_value() helper to do proper scaling taking
the fractionional values into account.

Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Hans de Goede <hansg@kernel.org>
---
Changes in v5:
- Introduce pval + pval2 local variables for better readability

Changes in v4:
- Use the new iio_multiply_value() helper
---
 drivers/iio/inkern.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/iio/inkern.c b/drivers/iio/inkern.c
index 158d54de14a7..1e5eb5a41271 100644
--- a/drivers/iio/inkern.c
+++ b/drivers/iio/inkern.c
@@ -726,20 +726,19 @@ int iio_read_channel_processed_scale(struct iio_channel *chan, int *val,
 				     unsigned int scale)
 {
 	struct iio_dev_opaque *iio_dev_opaque = to_iio_dev_opaque(chan->indio_dev);
-	int ret;
+	int ret, pval, pval2;
 
 	guard(mutex)(&iio_dev_opaque->info_exist_lock);
 	if (!chan->indio_dev->info)
 		return -ENODEV;
 
 	if (iio_channel_has_info(chan->channel, IIO_CHAN_INFO_PROCESSED)) {
-		ret = iio_channel_read(chan, val, NULL,
+		ret = iio_channel_read(chan, &pval, &pval2,
 				       IIO_CHAN_INFO_PROCESSED);
 		if (ret < 0)
 			return ret;
-		*val *= scale;
 
-		return ret;
+		return iio_multiply_value(val, scale, ret, pval, pval2);
 	} else {
 		ret = iio_channel_read(chan, val, NULL, IIO_CHAN_INFO_RAW);
 		if (ret < 0)
-- 
2.51.0