[PATCH] iio: hid-sensor: Fix scale for magnetic north heading

[Maximilian Czirr <maxx-cz@web.de>]

hid_sensor_format_scale() applies the HID unit exponent only when
the usage and unit match an entry in unit_conversion[].

Rotation-from-north support uses
HID_USAGE_SENSOR_ORIENT_COMP_MAGN_NORTH, but that usage has no entry in
the conversion table. As a result, the unit exponent is ignored and the
heading can be exposed with a scale of 1 instead of the scale specified
by the HID descriptor.

Add a unitless conversion entry so the HID unit exponent is applied to
the magnetic north heading. A sensor reporting an exponent of -5 then
correctly exposes a scale of 0.00001 instead of 1.

Fixes: abea9c62c32f ("iio: hid-sensor-magn-3d: Add support for rotation from north")
Cc: stable@vger.kernel.org
Signed-off-by: Maximilian Czirr <maxx-cz@web.de>
---
 drivers/iio/common/hid-sensors/hid-sensor-attributes.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/iio/common/hid-sensors/hid-sensor-attributes.c b/drivers/iio/common/hid-sensors/hid-sensor-attributes.c
index 9e7c26e37880..26f8554d18a5 100644
--- a/drivers/iio/common/hid-sensors/hid-sensor-attributes.c
+++ b/drivers/iio/common/hid-sensors/hid-sensor-attributes.c
@@ -38,6 +38,8 @@ static const struct {
 	{HID_USAGE_SENSOR_COMPASS_3D, 0, 0, 1000000},
 	{HID_USAGE_SENSOR_COMPASS_3D, HID_USAGE_SENSOR_UNITS_GAUSS, 1, 0},
 
+	{HID_USAGE_SENSOR_ORIENT_COMP_MAGN_NORTH, 0, 1, 0},
+
 	{HID_USAGE_SENSOR_INCLINOMETER_3D, 0, 0, 17453293},
 	{HID_USAGE_SENSOR_INCLINOMETER_3D,
 		HID_USAGE_SENSOR_UNITS_DEGREES, 0, 17453293},
-- 
2.56.0