[PATCH 1/2 v4] iio: Provide iio_read_channel_processed_scale() API

[Linus Walleij <linus.walleij@linaro.org>]

Since the old iio_read_channel_processed() would
lose precision if we fall back to reading raw and
scaling, we introduce a new API that will pass in
a scale factor when reading a processed channel:
iio_read_channel_processed_scale().

Refactor iio_read_channel_processed() as a special
case with scale factor 1.

Cc: Peter Rosin <peda@axentia.se>
Cc: Chris Lesiak <chris.lesiak@licor.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: linux-iio@vger.kernel.org
Link: https://lore.kernel.org/linux-iio/20201224011607.1059534-1-linus.walleij@linaro.org/
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
ChangeLog v3->v4:
- Split off as separate API-introducing patch.
- Fix a goto err when scaling.
- My suggestion is to apply this patch with Jonathan's ACK
  to the hwmon tree.
---
 drivers/iio/inkern.c         | 16 ++++++++++++++--
 include/linux/iio/consumer.h | 15 +++++++++++++++
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/drivers/iio/inkern.c b/drivers/iio/inkern.c
index db77a2d4a56b..c61fc06f98b8 100644
--- a/drivers/iio/inkern.c
+++ b/drivers/iio/inkern.c
@@ -688,7 +688,8 @@ int iio_read_channel_offset(struct iio_channel *chan, int *val, int *val2)
 }
 EXPORT_SYMBOL_GPL(iio_read_channel_offset);
 
-int iio_read_channel_processed(struct iio_channel *chan, int *val)
+int iio_read_channel_processed_scale(struct iio_channel *chan, int *val,
+				     unsigned int scale)
 {
 	int ret;
 
@@ -701,11 +702,15 @@ int iio_read_channel_processed(struct iio_channel *chan, int *val)
 	if (iio_channel_has_info(chan->channel, IIO_CHAN_INFO_PROCESSED)) {
 		ret = iio_channel_read(chan, val, NULL,
 				       IIO_CHAN_INFO_PROCESSED);
+		if (ret)
+			goto err_unlock;
+		*val *= scale;
 	} else {
 		ret = iio_channel_read(chan, val, NULL, IIO_CHAN_INFO_RAW);
 		if (ret < 0)
 			goto err_unlock;
-		ret = iio_convert_raw_to_processed_unlocked(chan, *val, val, 1);
+		ret = iio_convert_raw_to_processed_unlocked(chan, *val, val,
+							    scale);
 	}
 
 err_unlock:
@@ -713,6 +718,13 @@ int iio_read_channel_processed(struct iio_channel *chan, int *val)
 
 	return ret;
 }
+EXPORT_SYMBOL_GPL(iio_read_channel_processed_scale);
+
+int iio_read_channel_processed(struct iio_channel *chan, int *val)
+{
+	/* This is just a special case with scale factor 1 */
+	return iio_read_channel_processed_scale(chan, val, 1);
+}
 EXPORT_SYMBOL_GPL(iio_read_channel_processed);
 
 int iio_read_channel_scale(struct iio_channel *chan, int *val, int *val2)
diff --git a/include/linux/iio/consumer.h b/include/linux/iio/consumer.h
index 0a90ba8fa1bb..5fa5957586cf 100644
--- a/include/linux/iio/consumer.h
+++ b/include/linux/iio/consumer.h
@@ -241,6 +241,21 @@ int iio_read_channel_average_raw(struct iio_channel *chan, int *val);
  */
 int iio_read_channel_processed(struct iio_channel *chan, int *val);
 
+/**
+ * iio_read_channel_processed_scale() - read and scale a processed value
+ * @chan:		The channel being queried.
+ * @val:		Value read back.
+ * @scale:		Scale factor to apply during the conversion
+ *
+ * Returns an error code or 0.
+ *
+ * This function will read a processed value from a channel. This will work
+ * like @iio_read_channel_processed() but also scale with an additional
+ * scale factor while attempting to minimize any precision loss.
+ */
+int iio_read_channel_processed_scale(struct iio_channel *chan, int *val,
+				     unsigned int scale);
+
 /**
  * iio_write_channel_attribute() - Write values to the device attribute.
  * @chan:	The channel being queried.
-- 
2.29.2