[PATCH 0/3] iio: lsm303dlh-magn: endianness + boot-time fullscale selection

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

This series fixes two independent issues that together prevent the
LSM303DLH magnetometer from delivering usable readings out of the
box on at least the HP TouchPad (apq8060), and adds a small generic
extension to the ST sensors device-tree binding to allow boards to
declare a non-default initial full-scale.

PATCH 1/3 fixes st_sensors_core's read_axis_data() helper to honour
the channel's declared scan_type.endianness. The helper has
unconditionally decoded multi-byte results as little-endian since
it was introduced. Every other in-tree ST sensor declares IIO_LE
and was unaffected, but the LSM303DLH magnetometer publishes its
X / Y / Z words as big-endian pairs (high byte at the lower
register address, 0x03 / 0x05 / 0x07) and its channel specs in
st_magn_core.c correctly declare IIO_BE. The mismatch swapped the
high and low bytes of every magnetometer sample. The fix is generic
and only affects the IIO_BE branch; existing IIO_LE consumers are
untouched.

PATCH 2/3 adds an optional `st,fullscale-mg` device-tree property
to the ST sensors binding. The driver core hardcodes fs_avl[0] (the
highest-sensitivity range) as the starting full-scale, which is the
right default for a desk-noise floor but leaves no margin for
boards that pick up DC bias from nearby PCB structures. The
property lets a board declare the initial range in milligauss, with
the driver falling back to its built-in default on absence or on an
unsupported value. The property is purely additive and no in-tree
DTS is affected.

PATCH 3/3 parses st,fullscale-mg in the magnetometer common probe
and selects the matching fs_avl entry. The LSM303DLH on the HP
TouchPad picks up enough DC bias from the surrounding power planes
that the chip-default +/-1.3 G range saturates the X axis to the
chip's 0xF000 overflow sentinel on every sample, while Y and Z fall
within range. Empirically any fs_avl >= 1 (+/-1.9 G and up) works;
on tenderloin the appropriate value is 2500 mg (+/-2.5 G).

Before this series, on the HP TouchPad with the magnetometer flat
on a stable surface (sysfs in_magn_x_raw / in_magn_y_raw /
in_magn_z_raw, 15 reads at 200 ms intervals):

  X = 240             (high byte stuck at 0x00, low byte = 0xF0)
  Y = 12032..23296    (apparent random walk over ~11 K range)
  Z = -16128..-9728   (apparent random walk over ~6 K range)

After PATCH 1/3 alone (endianness fix), the chip reports its real
overflow sentinel for X:

  X = -4096           (= 0xF000, chip's saturation marker)
  Y =  91..103        (tight, sensible field reading)
  Z = 173..184        (tight, sensible field reading)

After the full series with st,fullscale-mg=2500 on tenderloin:

  X = -384..-403      (tight, sensible field reading)
  Y =   47..58
  Z =   96..108

Spans of 14..20 raw counts at scale 0.000670 G/LSB across all axes,
matching expected chip noise around the ambient DC bias.

PATCH 1/3 is a standalone bug fix and could be applied on its own
even if the binding extension is rejected. PATCHES 2/3 and 3/3 form
a unit.


Herman van Hazendonk (3):
  iio: common: st_sensors: honour channel endianness in read_axis_data
  dt-bindings: iio: st,st-sensors: add st,fullscale-mg
  iio: magnetometer: st_magn: honour st,fullscale-mg DT property

 .../bindings/iio/st,st-sensors.yaml           | 18 ++++++++++
 .../iio/common/st_sensors/st_sensors_core.c   | 19 +++++++---
 drivers/iio/magnetometer/st_magn_core.c       | 35 +++++++++++++++++++
 3 files changed, 67 insertions(+), 5 deletions(-)

-- 
2.43.0