[PATCH 2/2 v2] iio: magnetometer: ak8974: Provide scaling

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

The manual for the HSCDTD008A gives us a scaling for the
three axis as +/- 2.4mT per axis.

When I implement this the biggest axis indicates 0.59 Gauss
which is a reasonable measurement for the earths magnetic
which is in the range of 0.25 to 0.65 Gauss on the surface
according to Wikipedia.

Cc: Nick Reitemeyer <nick.reitemeyer@web.de>
Cc: Stephan Gerhold <stephan@gerhold.net>
Cc: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
ChangeLog v1->v2:
- Split out the measurement refactoring.
---
 drivers/iio/magnetometer/ak8974.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/iio/magnetometer/ak8974.c b/drivers/iio/magnetometer/ak8974.c
index 5361647b9054..effcdd93e650 100644
--- a/drivers/iio/magnetometer/ak8974.c
+++ b/drivers/iio/magnetometer/ak8974.c
@@ -603,6 +603,18 @@ static int ak8974_read_raw(struct iio_dev *indio_dev,
 		*val = outval;
 		ret = IIO_VAL_INT;
 		break;
+	case IIO_CHAN_INFO_SCALE:
+		/*
+		 * The datasheet for HSCDTF008A, page 3 specifies the
+		 * range of the sensor as +/- 2.4 mT per axis, which corresponds
+		 * to +/- 2400 uT = +/- 24 Gauss. So 0x7fff is 24 Gauss and
+		 * 0xffff is -24 Gauss. To account for the one missing value if
+		 * we multiply by 1/S16_MAX, instead multiply with 2/U16_MAX.
+		 */
+		*val = 24 * 2;
+		*val2 = U16_MAX;
+		ret = IIO_VAL_FRACTIONAL;
+		break;
 	}
 out_err_read:
 	return ret;
@@ -667,7 +679,8 @@ static const struct iio_chan_spec_ext_info ak8974_ext_info[] = {
 		.type = IIO_MAGN,					\
 		.modified = 1,						\
 		.channel2 = IIO_MOD_##axis,				\
-		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),		\
+		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |		\
+			BIT(IIO_CHAN_INFO_SCALE),			\
 		.ext_info = ak8974_ext_info,				\
 		.address = index,					\
 		.scan_index = index,					\
-- 
2.21.1