iio: make IIO_DMA_MINALIGN minimum of 8 bytes

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.

Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250507-iio-introduce-iio_declare_buffer_with_ts-v6-1-4aee1b9f1b89@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: dlech

include/linux/iio/iio.h

@@ -10,6 +10,7 @@
 #include <linux/device.h>
 #include <linux/cdev.h>
 #include <linux/compiler_types.h>
+#include <linux/minmax.h>
 #include <linux/slab.h>
 #include <linux/iio/types.h>
 /* IIO TODO LIST */
@@ -775,8 +776,14 @@ 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.
  */
-#define IIO_DMA_MINALIGN ARCH_DMA_MINALIGN
+#define IIO_DMA_MINALIGN MAX(ARCH_DMA_MINALIGN, sizeof(s64))
+
 struct iio_dev *iio_device_alloc(struct device *parent, int sizeof_priv);
 
 /* The information at the returned address is guaranteed to be cacheline aligned */