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

[David Lechner <dlechner@baylibre.com>]

On 1/22/26 2:29 PM, Jonathan Cameron wrote:
> 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.

Do we also need to add something to struct iio_scan_type to indicate
that the raw value is floating point as opposed to integer?

> 
> 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