Re: [PATCH v5 1/7] iio: make IIO_DMA_MINALIGN minimum of 8 bytes

[David Laight <david.laight.linux@gmail.com>]

On Mon, 05 May 2025 11:31:42 -0500
David Lechner <dlechner@baylibre.com> wrote:

> Add a condition to ensure that IIO_DMA_MINALIGN is at least 8 bytes.
> On some 32-bit architectures, IIO_DMA_MINALIGN is 4. In many cases,
> drivers are using this alignment for buffers that include a 64-bit
> timestamp that is used with iio_push_to_buffers_with_ts(), which expects
> the timestamp to be aligned to 8 bytes. To handle this, we can just make
> IIO_DMA_MINALIGN at least 8 bytes.
> 
> Signed-off-by: David Lechner <dlechner@baylibre.com>
> ---
>  include/linux/iio/iio.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h
> index 638cf2420fbd85cf2924d09d061df601d1d4bb2a..7e1e3739328d103262071bd34ba5f6631163c122 100644
> --- a/include/linux/iio/iio.h
> +++ b/include/linux/iio/iio.h
> @@ -775,8 +775,18 @@ static inline void *iio_device_get_drvdata(const struct iio_dev *indio_dev)
>   * to in turn include IIO_DMA_MINALIGN'd elements such as buffers which
>   * must not share  cachelines with the rest of the structure, thus making
>   * them safe for use with non-coherent DMA.
> + *
> + * A number of drivers also use this on buffers that include a 64-bit timestamp
> + * that is used with iio_push_to_buffer_with_ts(). Therefore, in the case where
> + * DMA alignment is not sufficient for proper timestamp alignment, we align to
> + * 8 bytes instead.
>   */
> +#if ARCH_DMA_MINALIGN < sizeof(s64)
> +#define IIO_DMA_MINALIGN sizeof(s64)
> +#else
>  #define IIO_DMA_MINALIGN ARCH_DMA_MINALIGN
> +#endif

Did you actually test this?
You can't use sizeof() in a pre-processor conditional.

	David

> +
>  struct iio_dev *iio_device_alloc(struct device *parent, int sizeof_priv);
>  
>  /* The information at the returned address is guaranteed to be cacheline aligned */
>