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

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

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