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

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

[Herman van Hazendonk]

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

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

[Herman van Hazendonk]

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.

Signed-off-by: Herman van Hazendonk <github.com@herrie.org>
---
 .../iio/common/st_sensors/st_sensors_core.c   | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/drivers/iio/common/st_sensors/st_sensors_core.c b/drivers/iio/common/st_sensors/st_sensors_core.c
index dbc5e16fbde4..84bce1539908 100644
--- a/drivers/iio/common/st_sensors/st_sensors_core.c
+++ b/drivers/iio/common/st_sensors/st_sensors_core.c
@@ -508,12 +508,21 @@ static int st_sensors_read_axis_data(struct iio_dev *indio_dev,
 	if (err < 0)
 		return err;
 
-	if (byte_for_channel == 1)
+	if (byte_for_channel == 1) {
 		*data = (s8)*outdata;
-	else if (byte_for_channel == 2)
-		*data = (s16)get_unaligned_le16(outdata);
-	else if (byte_for_channel == 3)
-		*data = (s32)sign_extend32(get_unaligned_le24(outdata), 23);
+	} else if (byte_for_channel == 2) {
+		if (ch->scan_type.endianness == IIO_BE)
+			*data = (s16)get_unaligned_be16(outdata);
+		else
+			*data = (s16)get_unaligned_le16(outdata);
+	} else if (byte_for_channel == 3) {
+		if (ch->scan_type.endianness == IIO_BE)
+			*data = (s32)sign_extend32(get_unaligned_be24(outdata),
+						   23);
+		else
+			*data = (s32)sign_extend32(get_unaligned_le24(outdata),
+						   23);
+	}
 
 	return 0;
 }
-- 
2.43.0

[PATCH 2/3] dt-bindings: iio: st,st-sensors: add st,fullscale-mg

[Herman van Hazendonk]

Add an optional st,fullscale-mg property that selects the initial
full-scale range of an ST MEMS sensor at probe time, expressed in
milligauss for magnetometers (and analogous engineering units for
other ST sensor families that may grow this property in the future).

The property is purely additive: if absent, drivers fall back to
their existing chip default, and if present but unsupported by the
specific sensor the driver warns and falls back. No existing in-tree
DTS is affected.

The motivating case is the LSM303DLH magnetometer on the HP TouchPad
(apq8060 / tenderloin) where the kernel's chip-default +/-1.3 G range
saturates the X axis to the chip's 0xF000 overflow sentinel out of
probe, because the chip is mounted close to surrounding power planes
and picks up enough DC bias to exceed the smallest range. The driver
core hardcodes fs_avl[0] as the starting range, so userspace cannot
recover without racing the driver to write the in_magn_x_scale sysfs
attribute after probe. st,fullscale-mg lets the device tree declare
a wider initial range up-front and avoids the race entirely.

Signed-off-by: Herman van Hazendonk <github.com@herrie.org>
---
 .../devicetree/bindings/iio/st,st-sensors.yaml | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/Documentation/devicetree/bindings/iio/st,st-sensors.yaml b/Documentation/devicetree/bindings/iio/st,st-sensors.yaml
index a1a958215cdb..335f38e9f78f 100644
--- a/Documentation/devicetree/bindings/iio/st,st-sensors.yaml
+++ b/Documentation/devicetree/bindings/iio/st,st-sensors.yaml
@@ -126,6 +126,24 @@ properties:
   mount-matrix:
     description: an optional 3x3 mounting rotation matrix.
 
+  st,fullscale-mg:
+    description: |
+      Selects the initial sensor full-scale at probe time, expressed in
+      milligauss for magnetometers (or analogous engineering units for
+      other sensor families that may grow this property in the future).
+      The value must match one of the sensor-specific full-scale ranges
+      supported by the chip; if the chip does not support the requested
+      range the driver falls back to its built-in default.
+
+      This is intended for boards where the magnetometer chip picks up
+      enough DC bias from nearby PCB structures (power planes, ferrous
+      shields, etc.) that the kernel's chip-default highest-sensitivity
+      range saturates one or more axes to the chip's overflow sentinel,
+      and userspace observes that axis as permanently stuck. Declaring
+      a wider initial range avoids the saturation at the cost of a
+      slightly coarser quantisation.
+    $ref: /schemas/types.yaml#/definitions/uint32
+
 allOf:
   - if:
       properties:
-- 
2.43.0

[PATCH 3/3] iio: magnetometer: st_magn: honour st,fullscale-mg DT property

[Herman van Hazendonk]

The ST magnetometer core's common probe hardcodes fs_avl[0] -- the
highest-sensitivity full-scale supported by the chip -- as the
starting range. For the LSM303DLH that is +/-1.3 G; for the
LSM303DLHC and LSM303DLM it is +/-2 G; for the LIS3MDL it is +/-4 G.

That is the right default for "minimal noise floor at a desk", but
it leaves no margin for boards that pick up appreciable DC bias from
nearby PCB structures. On the HP TouchPad (apq8060 / tenderloin) the
LSM303DLH magnetometer is mounted close enough to the surrounding
power planes that X reads back as the chip's 0xF000 overflow
sentinel (== -4096 raw, the value the chip publishes when the ADC
saturates) on every sample at the chip-default range, while Y and Z
fall well within the +/-1.3 G window.

Parse the st,fullscale-mg device-tree property (documented separately
in dt-bindings/iio/st,st-sensors.yaml) in the magnetometer common
probe to select the initial fs_avl entry by its mg value. The driver
tolerates an unknown / unsupported value by falling back to the chip
default and warning, so the property is purely additive -- existing
in-tree DTSes are unaffected.

Per-sensor mg ranges are listed in st_magn_sensors_settings[]. For
LSM303DLH the valid values are 1300, 1900, 2500, 4000, 4700, 5600
and 8100; for LSM303DLHC they are 1300, 1900, 2500, 4000, 4700, 5600,
8100 (same code path); for LIS3MDL they are 4000, 8000, 12000, 16000;
and so on. Sensors with a fixed full-scale (fs.addr == 0) simply
ignore the property.

Empirical scale sweep on the HP TouchPad confirmed that on this
board any fs_avl >= 1 produces non-saturated X readings:

    scale (0.001 G/LSB)  | X raw    Y raw    Z raw
    --------------------+-------------------------------
            1.100        | -4096    44       46    (X saturated)
            0.855        |  -547    37       37    (clean)
            0.670        |  -433    94      103    (clean)
            0.450        |  -266    44       71    (clean)
            0.400        |  -235    34       65    (clean)
            0.330        |  -196    27       56    (clean)
            0.230        |  -145    15       40    (clean)

2500 mg is the natural choice for tenderloin: comfortably outside
the saturation regime while keeping useful precision for compass
applications.

Signed-off-by: Herman van Hazendonk <github.com@herrie.org>
---
 drivers/iio/magnetometer/st_magn_core.c | 35 +++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/drivers/iio/magnetometer/st_magn_core.c b/drivers/iio/magnetometer/st_magn_core.c
index ef348d316c00..936253440856 100644
--- a/drivers/iio/magnetometer/st_magn_core.c
+++ b/drivers/iio/magnetometer/st_magn_core.c
@@ -10,6 +10,7 @@
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
+#include <linux/property.h>
 #include <linux/sysfs.h>
 #include <linux/iio/iio.h>
 #include <linux/iio/sysfs.h>
@@ -628,6 +629,40 @@ int st_magn_common_probe(struct iio_dev *indio_dev)
 	mdata->current_fullscale = &mdata->sensor_settings->fs.fs_avl[0];
 	mdata->odr = mdata->sensor_settings->odr.odr_avl[0].hz;
 
+	/*
+	 * Allow the device tree to override the default full-scale. Hardware
+	 * such as the LSM303DLH magnetometer on the HP TouchPad picks up
+	 * enough DC bias from nearby PCB structures that the chip-default
+	 * highest-sensitivity range saturates the X axis to a sentinel
+	 * 0xF000 immediately at probe; selecting a less sensitive range via
+	 * st,fullscale-mg fixes that without requiring userspace to write
+	 * in_magn_*_scale at startup.
+	 */
+	{
+		u32 fs_mg;
+
+		if (!device_property_read_u32(parent, "st,fullscale-mg",
+					      &fs_mg)) {
+			struct st_sensor_fullscale *fs =
+				&mdata->sensor_settings->fs;
+			int i;
+
+			for (i = 0; i < ST_SENSORS_FULLSCALE_AVL_MAX; i++) {
+				if (!fs->fs_avl[i].num)
+					break;
+				if (fs->fs_avl[i].num == fs_mg) {
+					mdata->current_fullscale =
+						&fs->fs_avl[i];
+					break;
+				}
+			}
+			if (mdata->current_fullscale->num != fs_mg)
+				dev_warn(parent,
+					 "st,fullscale-mg=%u not supported, using %u\n",
+					 fs_mg, mdata->current_fullscale->num);
+		}
+	}
+
 	if (!pdata)
 		pdata = (struct st_sensors_platform_data *)&default_magn_pdata;
 
-- 
2.43.0

Re: [PATCH 2/3] dt-bindings: iio: st,st-sensors: add st,fullscale-mg

[Conor Dooley]

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On Fri, Jun 05, 2026 at 12:08:42PM +0200, Herman van Hazendonk wrote:
> Add an optional st,fullscale-mg property that selects the initial
> full-scale range of an ST MEMS sensor at probe time, expressed in
> milligauss for magnetometers (and analogous engineering units for
> other ST sensor families that may grow this property in the future).
> 
> The property is purely additive: if absent, drivers fall back to
> their existing chip default, and if present but unsupported by the
> specific sensor the driver warns and falls back. No existing in-tree
> DTS is affected.
> 
> The motivating case is the LSM303DLH magnetometer on the HP TouchPad
> (apq8060 / tenderloin) where the kernel's chip-default +/-1.3 G range
> saturates the X axis to the chip's 0xF000 overflow sentinel out of
> probe, because the chip is mounted close to surrounding power planes
> and picks up enough DC bias to exceed the smallest range. The driver
> core hardcodes fs_avl[0] as the starting range, so userspace cannot
> recover without racing the driver to write the in_magn_x_scale sysfs
> attribute after probe. st,fullscale-mg lets the device tree declare
> a wider initial range up-front and avoids the race entirely.
> 
> Signed-off-by: Herman van Hazendonk <github.com@herrie.org>
> ---
>  .../devicetree/bindings/iio/st,st-sensors.yaml | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/iio/st,st-sensors.yaml b/Documentation/devicetree/bindings/iio/st,st-sensors.yaml
> index a1a958215cdb..335f38e9f78f 100644
> --- a/Documentation/devicetree/bindings/iio/st,st-sensors.yaml
> +++ b/Documentation/devicetree/bindings/iio/st,st-sensors.yaml
> @@ -126,6 +126,24 @@ properties:
>    mount-matrix:
>      description: an optional 3x3 mounting rotation matrix.
>  
> +  st,fullscale-mg:

I'd be inclined to say that this should spell out milligauss, but
this seems reasonable enough to me.
Acked-by: Conor Dooley <conor.dooley@microchip.com>
pw-bot: not-applicable

> +    description: |
> +      Selects the initial sensor full-scale at probe time, expressed in
> +      milligauss for magnetometers (or analogous engineering units for
> +      other sensor families that may grow this property in the future).
> +      The value must match one of the sensor-specific full-scale ranges
> +      supported by the chip; if the chip does not support the requested
> +      range the driver falls back to its built-in default.
> +
> +      This is intended for boards where the magnetometer chip picks up
> +      enough DC bias from nearby PCB structures (power planes, ferrous
> +      shields, etc.) that the kernel's chip-default highest-sensitivity
> +      range saturates one or more axes to the chip's overflow sentinel,
> +      and userspace observes that axis as permanently stuck. Declaring
> +      a wider initial range avoids the saturation at the cost of a
> +      slightly coarser quantisation.
> +    $ref: /schemas/types.yaml#/definitions/uint32
> +
>  allOf:
>    - if:
>        properties:
> -- 
> 2.43.0
> 

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