[PATCH v3 11/11] Documentation: iio: Document high-speed DMABUF based API

[Paul Cercueil]

Document the new DMABUF based API.

Signed-off-by: Paul Cercueil <paul@xxxxxxxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: linux-doc@xxxxxxxxxxxxxxx

---
v2: - Explicitly state that the new interface is optional and is
      not implemented by all drivers.
    - The IOCTLs can now only be called on the buffer FD returned by
      IIO_BUFFER_GET_FD_IOCTL.
    - Move the page up a bit in the index since it is core stuff and not
      driver-specific.
v3: Update the documentation to reflect the new API.
---
 Documentation/iio/dmabuf_api.rst | 59 ++++++++++++++++++++++++++++++++
 Documentation/iio/index.rst      |  2 ++
 2 files changed, 61 insertions(+)
 create mode 100644 Documentation/iio/dmabuf_api.rst

diff --git a/Documentation/iio/dmabuf_api.rst b/Documentation/iio/dmabuf_api.rst
new file mode 100644
index 000000000000..4d70372c7ebd
--- /dev/null
+++ b/Documentation/iio/dmabuf_api.rst
@@ -0,0 +1,59 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===================================
+High-speed DMABUF interface for IIO
+===================================
+
+1. Overview
+===========
+
+The Industrial I/O subsystem supports access to buffers through a
+file-based interface, with read() and write() access calls through the
+IIO device's dev node.
+
+It additionally supports a DMABUF based interface, where the userspace
+can attach DMABUF objects (externally created) to a IIO buffer, and
+subsequently use them for data transfers.
+
+A userspace application can then use this interface to share DMABUF
+objects between several interfaces, allowing it to transfer data in a
+zero-copy fashion, for instance between IIO and the USB stack.
+
+The userspace application can also memory-map the DMABUF objects, and
+access the sample data directly. The advantage of doing this vs. the
+read() interface is that it avoids an extra copy of the data between the
+kernel and userspace. This is particularly useful for high-speed devices
+which produce several megabytes or even gigabytes of data per second.
+It does however increase the userspace-kernelspace synchronization
+overhead, as the DMA_BUF_SYNC_START and DMA_BUF_SYNC_END IOCTLs have to
+be used for data integrity.
+
+2. User API
+===========
+
+As part of this interface, three new IOCTLs have been added. These three
+IOCTLs have to be performed on the IIO buffer's file descriptor,
+obtained using the IIO_BUFFER_GET_FD_IOCTL() ioctl.
+
+``IIO_BUFFER_DMABUF_ATTACH_IOCTL(int)``
+----------------------------------------------------------------
+
+Attach the DMABUF object, identified by its file descriptor, to the IIO
+buffer. Returns zero on success, and a negative errno value on error.
+
+``IIO_BUFFER_DMABUF_DETACH_IOCTL(int)``
+--------------------------------------------------------
+
+Detach the given DMABUF object, identified by its file descriptor, from
+the IIO buffer. Returns zero on success, and a negative errno value on
+error.
+
+Note that closing the IIO buffer's file descriptor will automatically
+detach all previously attached DMABUF objects.
+
+``IIO_BUFFER_DMABUF_ENQUEUE_IOCTL(struct iio_dmabuf *iio_dmabuf)``
+--------------------------------------------------------
+
+Enqueue a previously attached DMABUF object to the buffer queue.
+Enqueued DMABUFs will be read from (if output buffer) or written to
+(if input buffer) as long as the buffer is enabled.
diff --git a/Documentation/iio/index.rst b/Documentation/iio/index.rst
index 1b7292c58cd0..3eae8fcb1938 100644
--- a/Documentation/iio/index.rst
+++ b/Documentation/iio/index.rst
@@ -9,6 +9,8 @@ Industrial I/O
 
    iio_configfs
 
+   dmabuf_api
+
    ep93xx_adc
 
    bno055
-- 
2.39.2