linux-kernel - [PATCH v4 1/7] iio: introduce IIO_DECLARE_BUFFER_WITH

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Message-Id: <20250428-iio-introduce-iio_declare_buffer_with_ts-v4-1-6f7f6126f1cb@baylibre.com>
Date: Mon, 28 Apr 2025 15:23:03 -0500
From: David Lechner <dlechner@...libre.com>
To: Jonathan Cameron <jic23@...nel.org>, 
 David Lechner <dlechner@...libre.com>, 
 Nuno Sá <nuno.sa@...log.com>, 
 Andy Shevchenko <andy@...nel.org>, Lars-Peter Clausen <lars@...afoo.de>, 
 Michael Hennerich <Michael.Hennerich@...log.com>, 
 Eugen Hristev <eugen.hristev@...aro.org>, 
 Nicolas Ferre <nicolas.ferre@...rochip.com>, 
 Alexandre Belloni <alexandre.belloni@...tlin.com>, 
 Claudiu Beznea <claudiu.beznea@...on.dev>
Cc: linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-arm-kernel@...ts.infradead.org
Subject: [PATCH v4 1/7] 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().

Signed-off-by: David Lechner <dlechner@...libre.com>
---

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 | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h
index 638cf2420fbd85cf2924d09d061df601d1d4bb2a..95d90740b4ffb3ebd819d03ad2bd05ecf40d4384 100644
--- a/include/linux/iio/iio.h
+++ b/include/linux/iio/iio.h
@@ -7,9 +7,12 @@
 #ifndef _INDUSTRIAL_IO_H_
 #define _INDUSTRIAL_IO_H_
 
+#include <linux/align.h>
+#include <linux/build_bug.h>
 #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 */
@@ -777,6 +780,40 @@ 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) \
+	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)		\
+	/* IIO_DMA_MINALIGN may be 4 on some 32-bit arches. */	\
+	__aligned(MAX(IIO_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 */

-- 
2.43.0