iio: consumers: Add an iio_multiply_value() helper function

The channel-scale handling in iio_convert_raw_to_processed() in essence
does the following:

processed  = raw * caller-provided-scale * channel-scale

Which can also be written as:

multiplier = raw * caller-provided-scale
iio-value  = channel-scale
processed  = multiplier * iio-value

Where iio-value is a set of IIO_VAL_* type + val + val2 integers, being
able to handle multiplication of iio-values like this is something
which is useful to have in general and, as previous bugfixes to
iio_convert_raw_to_processed() have shown, also tricky to implement.

Split the iio-value multiplication code from iio_convert_raw_to_processed()
out into a new iio_multiply_value() helper. This serves multiple purposes:

1. Having this split out allows writing a KUnit test for this.
2. Having this split out allows re-use to get better precision
   when scaling values in iio_read_channel_processed_scale().

Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Hans de Goede <hansg@kernel.org>
Link: https://patch.msgid.link/20250831104825.15097-4-hansg@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/include/linux/iio/consumer.h b/include/linux/iio/consumer.h
index 6a4479616479..a38b277c2c02 100644
--- a/include/linux/iio/consumer.h
+++ b/include/linux/iio/consumer.h
@@ -382,6 +382,24 @@ int iio_read_channel_scale(struct iio_channel *chan, int *val,
 			   int *val2);
 
 /**
+ * iio_multiply_value() - Multiply an IIO value
+ * @result:	Destination pointer for the multiplication result
+ * @multiplier:	Multiplier.
+ * @type:	One of the IIO_VAL_* constants. This decides how the @val and
+ *		@val2 parameters are interpreted.
+ * @val:	Value being multiplied.
+ * @val2:	Value being multiplied. @val2 use depends on type.
+ *
+ * Multiply an IIO value with a s64 multiplier storing the result as
+ * IIO_VAL_INT. This is typically used for scaling.
+ *
+ * Returns:
+ * IIO_VAL_INT on success or a negative error-number on failure.
+ */
+int iio_multiply_value(int *result, s64 multiplier,
+		       unsigned int type, int val, int val2);
+
+/**
  * iio_convert_raw_to_processed() - Converts a raw value to a processed value
  * @chan:		The channel being queried
  * @raw:		The raw IIO to convert