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

[David Lechner <dlechner@baylibre.com>]

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.
> 
> Signed-off-by: David Lechner <dlechner@baylibre.com>
> ---
>  include/linux/iio/iio.h | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h
> index a9ecff191bd9..2c91b7659ce9 100644
> --- a/include/linux/iio/iio.h
> +++ b/include/linux/iio/iio.h
> @@ -931,6 +931,18 @@ static inline void *iio_device_get_drvdata(const struct iio_dev *indio_dev)
>  #define IIO_DECLARE_DMA_BUFFER_WITH_TS(type, name, count) \
>  	__IIO_DECLARE_BUFFER_WITH_TS(type, name, count) __aligned(IIO_DMA_MINALIGN)
>  
> +/**
> + * IIO_DECLARE_QUATERNION() - Declare a quaternion element
> + * @type: element type of the individual vectors
> + * @name: identifier name
> + *
> + * Quaternions are a vector composed of 4 elements (W, X, Y, Z). Use this macro
> + * to declare a quaternion element in a struct to ensure proper alignment in
> + * an IIO buffer.
> + */
> +#define IIO_DECLARE_QUATERNION(type, name) \
> +	type name[4] __aligned(sizeof(type) * 4)
> +
>  struct iio_dev *iio_device_alloc(struct device *parent, int sizeof_priv);
>  
>  /* The information at the returned address is guaranteed to be cacheline aligned */
> 

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.