Re: [PATCH v6 3/7] iio: imu: st_lsm6dsx: Fix check for invalid samples from FIFO

[Andy Shevchenko <andriy.shevchenko@intel.com>]

On Thu, Feb 26, 2026 at 10:43:34AM +0100, Geert Uytterhoeven wrote:
> On Wed, 25 Feb 2026 at 12:42, Andy Shevchenko
> <andriy.shevchenko@intel.com> wrote:
> > On Wed, Feb 25, 2026 at 11:17:11AM +0100, Francesco Lavra wrote:

...

> > > -     u8 iio_buff[ST_LSM6DSX_IIO_BUFF_SIZE] __aligned(8);
> > > +     struct {
> > > +             union {
> > > +                     __le16 data[3];
> > > +                     __le32 fifo_ts;
> > > +             };
> > > +             aligned_s64 timestamp;
> > > +     } iio_buff = { };
> >
> > Hmm... Have you considered m68k case? IIRC there the alignment is 2 (even for
> > 64-bit members), so theoretically the beginning of the structure can be aligned
> > to address 2. and hence the __le16 data[3] will have no gap to the following
> > timestamp. In current code it's 64-bit and not 48-bit item.
> 
> Fortunately, the layout of a structure does not change because the
> first member could be stored at a less-aligned address, as that would
> make it incompatible with a different instance that is stored at a
> differently-aligned address ;-)
> 
> The alignment of a structure is the maximum of the alignments of its
> members.  So that will be 8 bytes, as the aligned_s64 definition has
> an internal "__attribute__((aligned(8)))".

Thanks for the elaboration on this case, my knowledge about m68k is too shallow!

> It would be wise to add explicit padding ("u16 pad") just before
> timestamp though, for documentation purpose.

__le16 :-) Something like this:

     struct {
             union {
	             __le16 data[3];
                     __le32 fifo_ts;
             };
	     __le16 reserved; // ...compatibility with the previous impl...
             aligned_s64 timestamp;
     } iio_buff = { };

?

-- 
With Best Regards,
Andy Shevchenko