iio: buffer-dmaengine: document iio_dmaengine_buffer_setup_ext

The iio_dmaengine_buffer_setup_ext() function is public and should be
documented. Also, while touching this, fix the description of @dev in
related functions. @dev does not strictly have to be the parent of the
IIO device. It is only passed to dma_request_chan() so strictly
speaking, it can be any device that is a valid DMA channel consumer.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241115-dlech-mainline-spi-engine-offload-2-v5-9-bea815bd5ea5@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: dlech

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

@@ -255,7 +255,7 @@ static const struct iio_dev_attr *iio_dmaengine_buffer_attrs[] = {
 
 /**
  * iio_dmaengine_buffer_alloc() - Allocate new buffer which uses DMAengine
- * @dev: Parent device for the buffer
+ * @dev: DMA channel consumer device
  * @channel: DMA channel name, typically "rx".
  *
  * This allocates a new IIO buffer which internally uses the DMAengine framework
@@ -344,6 +344,21 @@ void iio_dmaengine_buffer_free(struct iio_buffer *buffer)
 }
 EXPORT_SYMBOL_NS_GPL(iio_dmaengine_buffer_free, IIO_DMAENGINE_BUFFER);
 
+/**
+ * iio_dmaengine_buffer_setup_ext() - Setup a DMA buffer for an IIO device
+ * @dev: DMA channel consumer device
+ * @indio_dev: IIO device to which to attach this buffer.
+ * @channel: DMA channel name, typically "rx".
+ * @dir: Direction of buffer (in or out)
+ *
+ * This allocates a new IIO buffer with devm_iio_dmaengine_buffer_alloc()
+ * and attaches it to an IIO device with iio_device_attach_buffer().
+ * It also appends the INDIO_BUFFER_HARDWARE mode to the supported modes of the
+ * IIO device.
+ *
+ * Once done using the buffer iio_dmaengine_buffer_free() should be used to
+ * release it.
+ */
 struct iio_buffer *iio_dmaengine_buffer_setup_ext(struct device *dev,
 						  struct iio_dev *indio_dev,
 						  const char *channel,
@@ -409,7 +424,7 @@ static void __devm_iio_dmaengine_buffer_free(void *buffer)
 
 /**
  * devm_iio_dmaengine_buffer_setup_ext() - Setup a DMA buffer for an IIO device
- * @dev: Parent device for the buffer
+ * @dev: Device for devm ownership and DMA channel consumer device
  * @indio_dev: IIO device to which to attach this buffer.
  * @channel: DMA channel name, typically "rx".
  * @dir: Direction of buffer (in or out)