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

[Andy Shevchenko]

On Fri, Jun 05, 2026 at 12:08:41PM +0200, Herman van Hazendonk wrote:
> 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.

...

> -	if (byte_for_channel == 1)
> +	if (byte_for_channel == 1) {
>  		*data = (s8)*outdata;

Maybe for the consistency's sake use sign_extend32() everywhere?

> +	} 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);

Why do you need casting here? sign_extend32() should return signed type.
With this being addressed, you can make them one-liners.

> +	}

	u32 tmp;
	...
	if (byte_for_channel == 1) {
		// this way is done to show the below variant
		tmp = *outdata;
		*data = sign_extend32(tmp, 7);
	} else if (byte_for_channel == 2) {
		if (ch->scan_type.endianness == IIO_BE)
			tmp = get_unaligned_be16(outdata);
		else
			tmp = get_unaligned_le16(outdata);
		*data = sign_extend32(tmp, 15);
	} else if (byte_for_channel == 3) {
		if (ch->scan_type.endianness == IIO_BE)
			tmp = get_unaligned_be24(outdata);
		else
			tmp = get_unaligned_le24(outdata);
		*data = sign_extend32(tmp, 23);
	}

Or even

	if (byte_for_channel == 1) {
		tmp = *outdata;
	} else if (byte_for_channel == 2) {
		if (ch->scan_type.endianness == IIO_BE)
			tmp = get_unaligned_be16(outdata);
		else
			tmp = get_unaligned_le16(outdata);
	} else if (byte_for_channel == 3) {
		if (ch->scan_type.endianness == IIO_BE)
			tmp = get_unaligned_be24(outdata);
		else
			tmp = get_unaligned_le24(outdata);
	} else {
		...error...
	}
	*data = sign_extend32(tmp, BYTES_TO_BITS(byte_for_channel) - 1);


-- 
With Best Regards,
Andy Shevchenko