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

[Jonathan Cameron]

On Wed, 17 Jun 2026 13:02:54 +0300
Andy Shevchenko <andriy.shevchenko@xxxxxxxxx> wrote:

> 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>
Applied to the fixes-togreg branch of iio.git. I'll wait until after
rc1 to send a pull request though (and rebase on rc1 in the meantime).
The other patches may have to wait, I haven't checked yet!

Jonathan

>