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.