iio: introduce IIO_DECLARE_BUFFER_WITH_TS macros

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().

Reviewed-by: Nuno Sá <[e-mail]>
Signed-off-by: David Lechner <[e-mail]>
Link: https://patch.msgid.link/202[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

include/linux/iio/iio.h

···
 #ifndef _INDUSTRIAL_IO_H_
 #define _INDUSTRIAL_IO_H_
 
+#include <linux/align.h>
 #include <linux/device.h>
 #include <linux/cdev.h>
 #include <linux/compiler_types.h>
···
  * 8 bytes instead.
  */
 #define IIO_DMA_MINALIGN MAX(ARCH_DMA_MINALIGN, sizeof(s64))
+
+#define __IIO_DECLARE_BUFFER_WITH_TS(type, name, count) \
+	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)
 
 struct iio_dev *iio_device_alloc(struct device *parent, int sizeof_priv);