Re: [PATCH v3 1/6] iio: introduce IIO_DECLARE_BUFFER_WITH_TS macros

[Jonathan Cameron <jic23@kernel.org>]

On Sat, 26 Apr 2025 17:34:10 -0500
David Lechner <dlechner@baylibre.com> wrote:

> On 4/26/25 6:35 AM, Jonathan Cameron wrote:
> > On Fri, 25 Apr 2025 16:08:43 -0500
> > David Lechner <dlechner@baylibre.com> wrote:
> >   
> 
> ...
> 
> >> @@ -777,6 +779,42 @@ static inline void *iio_device_get_drvdata(const struct iio_dev *indio_dev)
> >>   * them safe for use with non-coherent DMA.
> >>   */
> >>  #define IIO_DMA_MINALIGN ARCH_DMA_MINALIGN
> >> +
> >> +#define __IIO_DECLARE_BUFFER_WITH_TS(type, name, count) \
> >> +	static_assert(count); \  
> > 
> > Why do we care if count is 0?  Or is intent to check if is constant?
> > If the thought is we don't care either way about 0 (as rather nonsensical)
> > and this will fail to compile if not constant, then perhaps a comment would
> > avoid future confusion?  
> 
> I would be inclined to just leave out the check. But yes, it is just checking
> that count is constant and we don't expect 0.
> 
> >   
> >> +	type name[ALIGN((count), sizeof(s64) / sizeof(type)) + sizeof(s64) / sizeof(type)]
> >> +
> >> +/**
> >> + * IIO_DECLARE_BUFFER_WITH_TS() - Declare a buffer with timestamp
> >> + * @type: element type of the buffer
> >> + * @name: identifier name of the buffer
> >> + * @count: number of elements in the buffer
> >> + *
> >> + * Declares a buffer that is safe to use with iio_push_to_buffer_with_ts(). In
> >> + * addition to allocating enough space for @count elements of @type, it also
> >> + * allocates space for a s64 timestamp at the end of the buffer and ensures
> >> + * proper alignment of the timestamp.
> >> + */
> >> +#define IIO_DECLARE_BUFFER_WITH_TS(type, name, count) \
> >> +	__IIO_DECLARE_BUFFER_WITH_TS(type, name, count) __aligned(sizeof(s64))
> >> +
> >> +/**
> >> + * 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) __aligned(IIO_DMA_MINALIGN)
> >> +
> >> +static_assert(IIO_DMA_MINALIGN % sizeof(s64) == 0,  
> > That message isn't super helpful if seen in a compile log as we aren't reading the code here
> > "IIO_DECLARE_DMA_BUFFER_WITH_TS() assumes that ...
> >   
> >> +	"macros above assume that IIO_DMA_MINALIGN also ensures s64 timestamp alignment");
> >> +  
> 
> Seems we actually have an arch (openrisc) that triggers this [1]. This arch
> doesn't define ARCH_DMA_MINALIGN so it falls back to:
> 
> #define ARCH_DMA_MINALIGN __alignof__(unsigned long long)
> 
> Apparently this is only of those 32-bit arches that only does 4 byte alignment.
> From the official docs [2]:
> 
> 	Current OR32 implementations (OR1200) do not implement 8 byte alignment,
> 	but do require 4 byte alignment. Therefore the Application Binary
> 	Interface (chapter 16) uses 4 byte alignment for 8 byte types. Future
> 	extensions such as ORVDX64 may require natural alignment.
> 
> [1]: https://lore.kernel.org/linux-iio/20250425-iio-introduce-iio_declare_buffer_with_ts-v3-0-f12df1bff248@baylibre.com/T/#m91e0332673438793ff76949037ff40a34765ca30
> [2]: https://openrisc.io/or1k.html
> 
> 
> It looks like this could work (it compiles for me):
> 
> 	__aligned(MAX(IIO_DMA_MINALIGN, sizeof(s64)))
> 
> If that is OK we could leave out the static_assert(), unless we think there
> could be an arch with IIO_DMA_MINALIGN not a power of 2?!
> 
That change seems fine.  Non power of 2 arch would be fun but implausible any time soon :)