[PATCH v2 3/3] iio: magnetometer: st_magn: honour st,fullscale-milligauss 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-milligauss 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 DT binding pins the accepted value set per compatible
via allOf/if-then enum clauses, so a malformed mg value fails
dt_binding_check rather than reaching the driver. Sensors with a
fixed full-scale (fs.addr == 0: LSM303AGR, LIS2MDL, IIS2MDC) have no
register to switch and the property is rejected outright for them
in the binding; the parse block is additionally gated on fs.addr as
defence in depth against stale DTBs.

Per-sensor mg ranges are listed in st_magn_sensors_settings[]. For
LSM303DLH and LSM303DLHC/DLM the valid values are 1300, 1900, 2500,
4000, 4700, 5600 and 8100; for LIS3MDL, LSM9DS1-magn and LSM303C-magn
they are 4000, 8000, 12000, 16000.

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.

Assisted-by: Claude:claude-opus-4-7 sparse smatch clang-analyzer coccinelle checkpatch
Assisted-by: Sashiko:claude-opus-4-7
Signed-off-by: Herman van Hazendonk <github.com@xxxxxxxxxx>
---
 drivers/iio/magnetometer/st_magn_core.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/drivers/iio/magnetometer/st_magn_core.c b/drivers/iio/magnetometer/st_magn_core.c
index ef348d316c00..6f369e8dddea 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>
@@ -608,6 +609,7 @@ int st_magn_common_probe(struct iio_dev *indio_dev)
 	struct st_sensor_data *mdata = iio_priv(indio_dev);
 	struct device *parent = indio_dev->dev.parent;
 	struct st_sensors_platform_data *pdata = dev_get_platdata(parent);
+	const char *propname;
 	int err;
 
 	indio_dev->modes = INDIO_DIRECT_MODE;
@@ -628,6 +630,36 @@ 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;
 
+	/*
+	 * Skip fixed-FS chips (fs.addr == 0): no register to switch.
+	 * The binding rejects the property on these compatibles too;
+	 * the gate guards stale DTBs.
+	 */
+	propname = "st,fullscale-milligauss";
+	if (mdata->sensor_settings->fs.addr &&
+	    device_property_present(parent, propname)) {
+		struct st_sensor_fullscale *fs = &mdata->sensor_settings->fs;
+		u32 fs_mg;
+		int i;
+
+		err = device_property_read_u32(parent, propname, &fs_mg);
+		if (err)
+			return err;
+
+		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, "%s=%u not supported, using %u\n",
+				 propname, fs_mg,
+				 mdata->current_fullscale->num);
+	}
+
 	if (!pdata)
 		pdata = (struct st_sensors_platform_data *)&default_magn_pdata;
 

-- 
2.43.0