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

[David Lechner <dlechner@baylibre.com>]

On 3/16/26 2:03 PM, Francesco Lavra wrote:
> 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.

Makes sense.

> 
> 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.

I think you are referring to the 8-byte alignment for the timestamp?

Patch [1] should make a difference when the timestamp is not enabled
in a buffered read though.

When the timestamp is enabled, the buffer is going to be 16 bytes per
sample no matter what because of the 8-byte alignment of the timestamp.

But if the timestamp is not enabled, then for 16-bit storagebits and
repeat of 3, before the patch, the buffer would only be 6 bytes per
sample, but after the patch would be 8 bytes per sample. This doesn't
make a difference in the driver itself, but does make a difference to
userspace that is reading the buffer.

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