1.. SPDX-License-Identifier: GPL-2.0
2
3===================================
4High-speed DMABUF interface for IIO
5===================================
6
71. Overview
8===========
9
10The Industrial I/O subsystem supports access to buffers through a
11file-based interface, with read() and write() access calls through the
12IIO device's dev node.
13
14It additionally supports a DMABUF based interface, where the userspace
15can attach DMABUF objects (externally created) to an IIO buffer, and
16subsequently use them for data transfers.
17
18A userspace application can then use this interface to share DMABUF
19objects between several interfaces, allowing it to transfer data in a
20zero-copy fashion, for instance between IIO and the USB stack.
21
22The userspace application can also memory-map the DMABUF objects, and
23access the sample data directly. The advantage of doing this vs. the
24read() interface is that it avoids an extra copy of the data between the
25kernel and userspace. This is particularly useful for high-speed devices
26which produce several megabytes or even gigabytes of data per second.
27It does however increase the userspace-kernelspace synchronization
28overhead, as the DMA_BUF_SYNC_START and DMA_BUF_SYNC_END IOCTLs have to
29be used for data integrity.
30
312. User API
32===========
33
34As part of this interface, three new IOCTLs have been added. These three
35IOCTLs have to be performed on the IIO buffer's file descriptor, which
36can be obtained using the IIO_BUFFER_GET_FD_IOCTL() ioctl.
37
38  ``IIO_BUFFER_DMABUF_ATTACH_IOCTL(int fd)``
39    Attach the DMABUF object, identified by its file descriptor, to the
40    IIO buffer. Returns zero on success, and a negative errno value on
41    error.
42
43  ``IIO_BUFFER_DMABUF_DETACH_IOCTL(int fd)``
44    Detach the given DMABUF object, identified by its file descriptor,
45    from the IIO buffer. Returns zero on success, and a negative errno
46    value on error.
47
48    Note that closing the IIO buffer's file descriptor will
49    automatically detach all previously attached DMABUF objects.
50
51  ``IIO_BUFFER_DMABUF_ENQUEUE_IOCTL(struct iio_dmabuf *iio_dmabuf)``
52    Enqueue a previously attached DMABUF object to the buffer queue.
53    Enqueued DMABUFs will be read from (if output buffer) or written to
54    (if input buffer) as long as the buffer is enabled.
55