On Tue, 19 Dec 2023 18:50:09 +0100 Paul Cercueil <paul@xxxxxxxxxxxxxxx> wrote: > Document the new DMABUF based API. > > Signed-off-by: Paul Cercueil <paul@xxxxxxxxxxxxxxx> One minor comment inline. > > --- > 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. > --- > Documentation/iio/dmabuf_api.rst | 54 ++++++++++++++++++++++++++++++++ > Documentation/iio/index.rst | 2 ++ > 2 files changed, 56 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..1cd6cd51a582 > --- /dev/null > +++ b/Documentation/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 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. I would call out that they do not work on the main file descriptor (which is arguably also a IIO buffer file descriptor). > + > + ``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.
