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

[Andy Shevchenko]

On Fri, Jun 05, 2026 at 12:08:43PM +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-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.

...

> +	{

Oh, no. Use

	const char *propname;
	...
	propname = "st,fullscale-mg";
	if (device_property_present(..., propname)) {
		struct st_sensor_fullscale *fs = &mdata->sensor_settings->fs;
		u32 fs_mg;

		ret = device_property_read_u32(parent, propname, &fs_mg);
		if (ret)
			return ret;

		...
	}

instead.

> +		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;

This is strange. What's the point to go via the whole table? Does it have gaps?

> +				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",

Also use %s and propname.

> +					 fs_mg, mdata->current_fullscale->num);
> +		}
> +	}

-- 
With Best Regards,
Andy Shevchenko