Re: [PATCH v2 1/3] iio: common: st_sensors: honour channel endianness in read_axis_data

[Andy Shevchenko]

On Tue, Jun 16, 2026 at 03:02:04PM +0200, Herman van Hazendonk wrote:
> 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 LSM303DLHC
> and LSM303DLM share the same st_magn_16bit_channels (IIO_BE) and
> were therefore byte-swapped by the same bug; users of those parts
> will see different in_magn_*_raw values after this fix lands.
> 
> 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 (LSM303DLH 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.
> 
> While restructuring the branches, replace the previously implicit
> silent-success-with-uninitialised-*data fall-through for
> byte_for_channel outside 1..3 with an explicit return -EINVAL. No
> in-tree ST sensor publishes such a channel, but the new behaviour
> is strictly safer than handing userspace garbage.

Sounds like inevitable change in ABI, but worth doing it.
Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxx>

-- 
With Best Regards,
Andy Shevchenko