[PATCH 1/3] iio: common: st_sensors: honour channel endianness in read_axis_data

[Herman van Hazendonk <github.com@herrie.org>]

st_sensors_read_axis_data() unconditionally decoded multi-byte
results with get_unaligned_le16() / get_unaligned_le24() regardless
of the channel's declared scan_type.endianness.

For every ST sensor that has used this helper since it was introduced
this happened to be fine because the ST IMU/accel/gyro/pressure
families publish their data registers as little-endian and the
channel specs in those drivers declare IIO_LE accordingly.

The LSM303DLH magnetometer however publishes its X/Y/Z output as a
pair of big-endian bytes (the H register sits at the lower address,
0x03/0x05/0x07, and the L register immediately after), and its
channel specs in st_magn_core.c correctly declare IIO_BE -- but
read_axis_data() ignored that and decoded as little-endian, swapping
the high and low bytes of every magnetometer sample.

The bug is most visible on a stationary chip: in earth's field the
true X reading is small and the high byte sits at 0x00, so swapping
the bytes pins sysfs X at exactly the low byte's pattern (e.g. 0x00F0
= 240). Y and Z still appear "to vary" because their magnitudes are
larger and the noise in the low byte produces big swings in the
swapped high byte:

  before (chip flat, sysfs in_magn_*_raw):
      X=240 (stuck), Y= 12032..23296, Z=-16128..-9728

  after (direct i2c-dev big-endian decode, same chip same orientation):
      X≈-4096, Y≈210, Z≈80     (sensible values reflecting earth's
                                ambient field at low gauss range)

Fix read_axis_data() to dispatch on ch->scan_type.endianness and
call get_unaligned_be16() / get_unaligned_be24() when the channel
declares IIO_BE. Existing IIO_LE consumers (st_accel, st_gyro,
st_pressure, st_lsm6dsx and others) are unaffected because their
channel specs already declare IIO_LE and the LE path is unchanged.

Signed-off-by: Herman van Hazendonk <github.com@herrie.org>
---
 .../iio/common/st_sensors/st_sensors_core.c   | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/drivers/iio/common/st_sensors/st_sensors_core.c b/drivers/iio/common/st_sensors/st_sensors_core.c
index dbc5e16fbde4..84bce1539908 100644
--- a/drivers/iio/common/st_sensors/st_sensors_core.c
+++ b/drivers/iio/common/st_sensors/st_sensors_core.c
@@ -508,12 +508,21 @@ static int st_sensors_read_axis_data(struct iio_dev *indio_dev,
 	if (err < 0)
 		return err;
 
-	if (byte_for_channel == 1)
+	if (byte_for_channel == 1) {
 		*data = (s8)*outdata;
-	else if (byte_for_channel == 2)
-		*data = (s16)get_unaligned_le16(outdata);
-	else if (byte_for_channel == 3)
-		*data = (s32)sign_extend32(get_unaligned_le24(outdata), 23);
+	} else if (byte_for_channel == 2) {
+		if (ch->scan_type.endianness == IIO_BE)
+			*data = (s16)get_unaligned_be16(outdata);
+		else
+			*data = (s16)get_unaligned_le16(outdata);
+	} else if (byte_for_channel == 3) {
+		if (ch->scan_type.endianness == IIO_BE)
+			*data = (s32)sign_extend32(get_unaligned_be24(outdata),
+						   23);
+		else
+			*data = (s32)sign_extend32(get_unaligned_le24(outdata),
+						   23);
+	}
 
 	return 0;
 }
-- 
2.43.0