Re: [PATCH v5 4/4] iio: imu: st_lsm6dsx: Add support for rotation sensor

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 22 Jan 2026 17:23:35 +0100
Francesco Lavra <flavra@baylibre.com> wrote:

> Some IMU chips in the LSM6DSX family have sensor fusion features that
> combine data from the accelerometer and gyroscope. One of these features
> generates rotation vector data and makes it available in the hardware
> FIFO as a quaternion (more specifically, the X, Y and Z components of the
> quaternion vector, expressed as 16-bit half-precision floating-point
> numbers).
> 
> Add support for a new sensor instance that allows receiving sensor fusion
> data, by defining a new struct st_lsm6dsx_sf_settings (which contains
> chip-specific details for the sensor fusion functionality), and adding this
> struct as a new field in struct st_lsm6dsx_settings. In st_lsm6dsx_core.c,
> populate this new struct for the LSM6DSV and LSM6DSV16X chips, and add the
> logic to initialize an additional IIO device if this struct is populated
> for the hardware type being probed.
> Note: a new IIO device is being defined (as opposed to adding channels to
> an existing device) because the rate at which sensor fusion data is
> generated may not match the data rate from any of the existing devices.
> 
> Tested on LSMDSV16X.
> 
> Signed-off-by: Francesco Lavra <flavra@baylibre.com>
> Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>

> diff --git a/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_buffer.c b/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_buffer.c
> index ded9a96076e6..3b4fa57bf461 100644
> --- a/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_buffer.c
> +++ b/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_buffer.c

> @@ -580,6 +584,16 @@ st_lsm6dsx_push_tagged_data(struct st_lsm6dsx_hw *hw, u8 tag,
>  	case ST_LSM6DSX_EXT2_TAG:
>  		iio_dev = hw->iio_devs[ST_LSM6DSX_ID_EXT2];
>  		break;
> +	case ST_LSM6DSX_ROT_TAG:
> +		/*
> +		 * The sensor reports only the {X, Y, Z} elements of the
> +		 * quaternion vector; set the W value to 0 (it can be derived
> +		 * from the {X, Y, Z} values due to the property that the vector
> +		 * is normalized).

I'd missed this before.  This is going to really confuse user space.
I don't think we can just return it with a 0 in that last entry.
At the very least we need an ABI doc update to reflect this oddity.

I don't think that is enough though. This isn't a quaternion, but
rather something we can derive one from. Annoying though it is,
we can't realistically fix it up in kernel, so we are probably talking
a new MOD_TYPE.

Also it's been a long time since I did much with quaternions,
but isn't the sign of w ambiguous if we are relying on only X, Y and Z?
A bit of googling + AI suggests flipping it inverts the direction of
rotation around a given axis. Feels like there is a constraint missing
in this description.

Jonathan