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

[me]

On 2026-06-14 20:39, Jonathan Cameron wrote:
> On Fri,  5 Jun 2026 12:08:41 +0200
> Herman van Hazendonk <github.com@xxxxxxxxxx> 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 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.
> Hi Herman,
>
> Fixes tag please.   We want to know how far to backport and provide
> info to those trying to work out if the bug affects their kernel
> trees.
>
> Thanks
>
> Jonathan
>
> > Signed-off-by: Herman van Hazendonk <github.com@xxxxxxxxxx>
Hi Jonathan,

I've been working with the HP TouchPad which uses the Qualcomm APQ8060 
to bring this to mainline kernel, which was running a very ancient 
2.6.35 kernel where I could compare things since it was working there 
without issues, hence I could compare them side by side, check values 
etc.

It seems this "bug" was introduced with 3.9:

Fixes: 23491b513bcd ("iio:common: Add STMicroelectronics common 
library")

That's the commit that introduced st_sensors_read_axis_data() with the 
unconditional get_unaligned_le16() decode.

I'll include that for the v2.

Thanks,
Herman