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

[me@herrie.org]

On 2026-06-14 20:39, Jonathan Cameron wrote:
> On Fri,  5 Jun 2026 12:08:41 +0200
> Herman van Hazendonk <github.com@herrie.org> 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@herrie.org>
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