Re: [PATCH v4 1/7] iio: introduce IIO_DECLARE_BUFFER_WITH_TS macros

["Nuno Sá" <noname.nuno@gmail.com>]

On Tue, 2025-04-29 at 14:31 -0500, David Lechner wrote:
> On 4/28/25 9:12 PM, David Lechner wrote:
> > On 4/28/25 3:23 PM, David Lechner wrote:
> > > Add new macros to help with the common case of declaring a buffer that
> > > is safe to use with iio_push_to_buffers_with_ts(). This is not trivial
> > > to do correctly because of the alignment requirements of the timestamp.
> > > This will make it easier for both authors and reviewers.
> > > 
> > > To avoid double __align() attributes in cases where we also need DMA
> > > alignment, add a 2nd variant IIO_DECLARE_DMA_BUFFER_WITH_TS().
> > > 
> > > Signed-off-by: David Lechner <dlechner@baylibre.com>
> > > ---
> > 
> > ...
> > 
> > > +/**
> > > + * IIO_DECLARE_DMA_BUFFER_WITH_TS() - Declare a DMA-aligned buffer with
> > > timestamp
> > > + * @type: element type of the buffer
> > > + * @name: identifier name of the buffer
> > > + * @count: number of elements in the buffer
> > > + *
> > > + * Same as IIO_DECLARE_BUFFER_WITH_TS(), but is uses
> > > __aligned(IIO_DMA_MINALIGN)
> > > + * to ensure that the buffer doesn't share cachelines with anything that
> > > comes
> > > + * before it in a struct. This should not be used for stack-allocated
> > > buffers
> > > + * as stack memory cannot generally be used for DMA.
> > > + */
> > > +#define IIO_DECLARE_DMA_BUFFER_WITH_TS(type, name, count)	\
> > > +	__IIO_DECLARE_BUFFER_WITH_TS(type, name, count)		\
> > > +	/* IIO_DMA_MINALIGN may be 4 on some 32-bit arches. */	\
> > > +	__aligned(MAX(IIO_DMA_MINALIGN, sizeof(s64)))
> > 
> > I just realized my logic behind this is faulty. It assumes sizeof(s64) ==
> > __alignof__(s64), but that isn't always true and that is what caused the
> > builds
> > to hit the static_assert() on v3.
> > 
> > We should be able to leave this as __aligned(IIO_DMA_MINALIGN)
> > 
> > And have this (with better error message):
> > 
> > static assert(IIO_DMA_MINALIGN % __alignof__(s64) == 0);
> 
> I was working late yesterday and should have saved that reply until morning
> to think about it more!
> 
> We do want to align to to sizeof(s64) instead of __alignof__(s64) to avoid
> issues with, e.g. 32-bit kernel and 64-bit userspace (same reason that
> aligned_s64 exists and always uses 8-byte alignment).

What issues could we have? In your inner macros I think you still make sure we
pad everything up to sizeof(s64) right? Am I missing any subtle issue?

but...

> 
> So I think this patch is correct as-is after all.

FWIW, I do prefer this approach or what Andy suggest (min as sizeof(s64)).

- Nuno Sá