iio: buffer-dmaengine: Report buffer length requirements

The dmaengine buffer has some length alignment requirements that can differ
from platform to platform. If the length alignment requirements are not met
unexpected behavior like dropping of samples can occur.

Currently these requirements are not reported and applications need to know
the requirements of the platform by some out-of-band means.

Add a new buffer attribute that reports the length alignment requirements
called `length_align_bytes`. The reported length alignment is in bytes that
means the buffer length alignment in sample sets depends on the number of
enabled channels and the bytes per channel. Applications using this
attribute to determine the buffer size requirements need to consider this.

Signed-off-by: Lars-Peter Clausen <[e-mail]>
Signed-off-by: Alexandru Ardelean <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/ABI/testing/sysfs-bus-iio-dma-buffer

···
+What:		/sys/bus/iio/devices/iio:deviceX/buffer/length_align_bytes
+KernelVersion:	5.4
+Contact:	[e-mail]
+Description:
+		DMA buffers tend to have a alignment requirement for the
+		buffers. If this alignment requirement is not met samples might
+		be dropped from the buffer.
+
+		This property reports the alignment requirements in bytes.
+		This means that the buffer size in bytes needs to be a integer
+		multiple of the number reported by this file.
+
+		The alignment requirements in number of sample sets will depend
+		on the enabled channels and the bytes per channel. This means
+		that the alignment requirement in samples sets might change
+		depending on which and how many channels are enabled. Whereas
+		the alignment requirement reported in bytes by this property
+		will remain static and does not depend on which channels are
+		enabled.

drivers/iio/buffer/industrialio-buffer-dmaengine.c

···
 #include <linux/module.h>
 
 #include <linux/iio/iio.h>
+#include <linux/iio/sysfs.h>
 #include <linux/iio/buffer.h>
 #include <linux/iio/buffer_impl.h>
 #include <linux/iio/buffer-dma.h>
···
 	.abort = iio_dmaengine_buffer_abort,
 };
 
+static ssize_t iio_dmaengine_buffer_get_length_align(struct device *dev,
+	struct device_attribute *attr, char *buf)
+{
+	struct iio_dev *indio_dev = dev_to_iio_dev(dev);
+	struct dmaengine_buffer *dmaengine_buffer =
+		iio_buffer_to_dmaengine_buffer(indio_dev->buffer);
+
+	return sprintf(buf, "%u\n", dmaengine_buffer->align);
+}
+
+static IIO_DEVICE_ATTR(length_align_bytes, 0444,
+		       iio_dmaengine_buffer_get_length_align, NULL, 0);
+
+static const struct attribute *iio_dmaengine_buffer_attrs[] = {
+	&iio_dev_attr_length_align_bytes.dev_attr.attr,
+	NULL,
+};
+
 /**
  * iio_dmaengine_buffer_alloc() - Allocate new buffer which uses DMAengine
  * @dev: Parent device for the buffer
···
 
 	iio_dma_buffer_init(&dmaengine_buffer->queue, chan->device->dev,
 		&iio_dmaengine_default_ops);
+	iio_buffer_set_attrs(&dmaengine_buffer->queue.buffer,
+		iio_dmaengine_buffer_attrs);
 
 	dmaengine_buffer->queue.buffer.access = &iio_dmaengine_buffer_ops;