[PATCH v6 2/7] iio: introduce IIO_DECLARE_BUFFER_WITH_TS macros

[David Lechner]

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 <nuno.sa at analog.com>
Signed-off-by: David Lechner <dlechner at baylibre.com>
---

v5 changes:
* Revert back to __align(IIO_DMA_MINALIGN) as IIO_DMA_MINALIGN now has
  a minimum of 8 bytes.

v4 changes:
* Drop the static_asserts(). Some 32-bit arches were triggering one, so
  we had to address the problem instead of hoping that it didn't exist.
  The other made a multi-statement macro, which isn't the best practice
  and didn't have a way to make a really helpful error message. The
  condition we were trying to catch is still caught by -Wvla.
* Changed __align(IIO_DMA_MINALIGN) to __align(MAX(IIO_DMA_MINALIGN,
  sizeof(s64))). As the build-bots found, there are some 32-bit arches
  where IIO_DMA_MINALIGN is 4, but we still need 8-byte alignment for
  the timestamp.

v3 changes:
* Use leading double-underscore for "private" macro to match "private"
  functions that do the same.
* Use static_assert() from linux/build_bug.h instead of _Static_assert()
* Fix incorrectly using sizeof(IIO_DMA_MINALIGN).
* Add check that count argument is constant. (Note, I didn't include a
  message in this static assert because it already gives a reasonable
  message.)

/home/david/work/bl/linux/drivers/iio/accel/sca3300.c:482:51: error: expression in static assertion is not constant
  482 |         IIO_DECLARE_BUFFER_WITH_TS(s16, channels, val);
      |                                                   ^~~

v2 changes:
* Add 2nd macro for DMA alignment
---
 include/linux/iio/iio.h | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h
index a574f22398e45cad1ea741d20d302f88756a1b13..d11668f14a3e17654fcf17a4e853d4b493205019 100644
--- a/include/linux/iio/iio.h
+++ b/include/linux/iio/iio.h
@@ -7,6 +7,7 @@
 #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>
@@ -784,6 +785,37 @@ static inline void *iio_device_get_drvdata(const struct iio_dev *indio_dev)
  */
 #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);
 
 /* The information at the returned address is guaranteed to be cacheline aligned */

-- 
2.43.0