Re: [PATCH v2 3/3] iio: magnetometer: st_magn: honour st,fullscale-milligauss DT property

[Andy Shevchenko]

On Tue, Jun 16, 2026 at 03:02:06PM +0200, Herman van Hazendonk wrote:
> 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.

Not sure if we need that big commit message, better to move to the point.

> +	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;

Instead...

> +		err = device_property_read_u32(parent, propname, &fs_mg);
> +		if (err)
> +			return err;

> +		for (i = 0; i < ST_SENSORS_FULLSCALE_AVL_MAX; i++) {

		for (unsigned int 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);
> +	}

-- 
With Best Regards,
Andy Shevchenko