iio: Add iio_push_buffers_with_timestamp() helper

Drivers using software buffers often store the timestamp in their data buffer
before calling iio_push_to_buffers() with that data buffer. Storing the
timestamp in the buffer usually involves some ugly pointer arithmetic. This
patch adds a new helper function called iio_push_buffers_with_timestamp() which
is similar to iio_push_to_buffers but takes an additional timestamp parameter.
The function will help to hide to uglyness in one central place instead of
exposing it in every driver. If timestamps are enabled for the IIO device
iio_push_buffers_with_timestamp() will store the timestamp as the last element
in buffer, before passing the buffer on to iio_push_buffers(). The buffer needs
large enough to hold the timestamp in this case. If timestamps are disabled
iio_push_buffers_with_timestamp() will behave just like iio_push_buffers().

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: Oleksandr Kravchenko <o.v.kravchenko@globallogic.com>
Cc: Josh Wu <josh.wu@atmel.com>
Cc: Denis Ciocca <denis.ciocca@gmail.com>
Cc: Manuel Stahl <manuel.stahl@iis.fraunhofer.de>
Cc: Ge Gao <ggao@invensense.com>
Cc: Peter Meerwald <pmeerw@pmeerw.net>
Cc: Jacek Anaszewski <j.anaszewski@samsung.com>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

include/linux/iio/buffer.h

@@ -122,6 +122,31 @@ int iio_scan_mask_set(struct iio_dev *indio_dev,
  */
 int iio_push_to_buffers(struct iio_dev *indio_dev, const void *data);
 
+/*
+ * iio_push_to_buffers_with_timestamp() - push data and timestamp to buffers
+ * @indio_dev:		iio_dev structure for device.
+ * @data:		sample data
+ * @timestamp:		timestamp for the sample data
+ *
+ * Pushes data to the IIO device's buffers. If timestamps are enabled for the
+ * device the function will store the supplied timestamp as the last element in
+ * the sample data buffer before pushing it to the device buffers. The sample
+ * data buffer needs to be large enough to hold the additional timestamp
+ * (usually the buffer should be indio->scan_bytes bytes large).
+ *
+ * Returns 0 on success, a negative error code otherwise.
+ */
+static inline int iio_push_to_buffers_with_timestamp(struct iio_dev *indio_dev,
+	void *data, int64_t timestamp)
+{
+	if (indio_dev->scan_timestamp) {
+		size_t ts_offset = indio_dev->scan_bytes / sizeof(int64_t) - 1;
+		((int64_t *)data)[ts_offset] = timestamp;
+	}
+
+	return iio_push_to_buffers(indio_dev, data);
+}
+
 int iio_update_demux(struct iio_dev *indio_dev);
 
 /**