[PATCH v12 6/7] Documentation: iio: Document high-speed DMABUF based API

[Paul Cercueil <paul@crapouillou.net>]

Document the new DMABUF based API.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Co-developed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Nuno Sa <nuno.sa@analog.com>

---
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.

v5: Use description lists for the documentation of the three new IOCTLs
    instead of abusing subsections.

v8: Renamed dmabuf_api.rst -> iio_dmabuf_api.rst, and updated index.rst
    whose format changed in iio/togreg.

v11:
- "a IIO buffer" -> "an IIO buffer"
- Add variable name in IOCTL calls

v12:
- "obtained using..." -> "which can be obtained using..."
---
 Documentation/iio/iio_dmabuf_api.rst | 54 ++++++++++++++++++++++++++++
 Documentation/iio/index.rst          |  1 +
 2 files changed, 55 insertions(+)
 create mode 100644 Documentation/iio/iio_dmabuf_api.rst

diff --git a/Documentation/iio/iio_dmabuf_api.rst b/Documentation/iio/iio_dmabuf_api.rst
new file mode 100644
index 000000000000..2836cadbd495
--- /dev/null
+++ b/Documentation/iio/iio_dmabuf_api.rst
@@ -0,0 +1,54 @@
+.. 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 an 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, which
+can be obtained using the IIO_BUFFER_GET_FD_IOCTL() ioctl.
+
+  ``IIO_BUFFER_DMABUF_ATTACH_IOCTL(int fd)``
+    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 fd)``
+    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 4c13bfa2865c..9cb4c50cb20d 100644
--- a/Documentation/iio/index.rst
+++ b/Documentation/iio/index.rst
@@ -9,6 +9,7 @@ Industrial I/O
 
    iio_configfs
    iio_devbuf
+   iio_dmabuf_api
    iio_tools
 
 Industrial I/O Kernel Drivers
-- 
2.43.0