Re: [PATCH v2 1/2] iio: add IIO_DECLARE_QUATERNION() macro

[Francesco Lavra]

On Sat, 2026-03-07 at 18:35 -0600, David Lechner wrote:
> On 2/28/26 2:02 PM, David Lechner wrote:
> > Add a new IIO_DECLARE_QUATERNION() macro that is used to declare the
> > field in an IIO buffer struct that contains a quaternion vector.
> > 
> > Quaternions are currently the only IIO data type that uses the .repeat
> > feature of struct iio_scan_type. This has an implicit rule that the
> > element in the buffer must be aligned to the entire size of the
> > repeated
> > element. This macro will make that requirement explicit. Since this is
> > the only user, we just call the macro IIO_DECLARE_QUATERNION() instead
> > of something more generic.

...


> Hi Francesco,
> 
> I should have cc'ed you on this one. We'll want to add another macro
> like this for IIO_DECLARE_QUATERNION_AXIS(), I imagine. (This has been
> applied to the fixes-togreg branch since it is a dependency to a fix.)
> 
> What to do on that for the alignment though is an open question since
> we've never had a repeat of 3 before. The question is if it is OK to
> have an alignment that isn't a power of two bytes. For your case, since
> the data is 3 x 16-bit, it would be 3 * 2 = 6 bytes.

Hi David. Differently from the hid-sensor driver (where the `scan` field in
struct dev_rot_state is used exclusively for quaternion data), the lsm6dsx
driver uses a data structure (see the `iio_buff` variable in
st_lsm6dsx_buffer.c:st_lsm6dsx_read_tagged_fifo()) that is shared between
all the different data types that can be read from the hardware FIFO
(accel, gyro, quaternion, external sensor data), so changing this structure
to a quaternion-specific type is not ideal. So I think for the time being
there wouldn't be any users of an IIO_DECLARE_QUATERNION_AXIS() macro.

As for the alignment, according to your patch at [1], when the repeat
number is not a power of two, it is (will be) rounded up to the next power
of two (and this is consistent with what the lsm6dsx driver expects), so
the alignment will be 8 bytes.

[1]
https://lore.kernel.org/linux-iio/20260307-iio-fix-timestamp-alignment-v2-4-d1d48fbadbbf@xxxxxxxxxxxx/