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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> Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com> Link: https://patch.msgid.link/20240620122726.41232-7-paul@crapouillou.net Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
55 lines
2.2 KiB
ReStructuredText
55 lines
2.2 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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===================================
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High-speed DMABUF interface for IIO
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===================================
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1. Overview
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===========
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The Industrial I/O subsystem supports access to buffers through a
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file-based interface, with read() and write() access calls through the
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IIO device's dev node.
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It additionally supports a DMABUF based interface, where the userspace
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can attach DMABUF objects (externally created) to an IIO buffer, and
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subsequently use them for data transfers.
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A userspace application can then use this interface to share DMABUF
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objects between several interfaces, allowing it to transfer data in a
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zero-copy fashion, for instance between IIO and the USB stack.
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The userspace application can also memory-map the DMABUF objects, and
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access the sample data directly. The advantage of doing this vs. the
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read() interface is that it avoids an extra copy of the data between the
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kernel and userspace. This is particularly useful for high-speed devices
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which produce several megabytes or even gigabytes of data per second.
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It does however increase the userspace-kernelspace synchronization
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overhead, as the DMA_BUF_SYNC_START and DMA_BUF_SYNC_END IOCTLs have to
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be used for data integrity.
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2. User API
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===========
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As part of this interface, three new IOCTLs have been added. These three
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IOCTLs have to be performed on the IIO buffer's file descriptor, which
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can be obtained using the IIO_BUFFER_GET_FD_IOCTL() ioctl.
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``IIO_BUFFER_DMABUF_ATTACH_IOCTL(int fd)``
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Attach the DMABUF object, identified by its file descriptor, to the
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IIO buffer. Returns zero on success, and a negative errno value on
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error.
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``IIO_BUFFER_DMABUF_DETACH_IOCTL(int fd)``
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Detach the given DMABUF object, identified by its file descriptor,
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from the IIO buffer. Returns zero on success, and a negative errno
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value on error.
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Note that closing the IIO buffer's file descriptor will
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automatically detach all previously attached DMABUF objects.
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``IIO_BUFFER_DMABUF_ENQUEUE_IOCTL(struct iio_dmabuf *iio_dmabuf)``
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Enqueue a previously attached DMABUF object to the buffer queue.
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Enqueued DMABUFs will be read from (if output buffer) or written to
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(if input buffer) as long as the buffer is enabled.
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