Paul Cercueil paul@crapouillou.net writes:
One nit:
Document the new DMABUF based API.
Signed-off-by: Paul Cercueil paul@crapouillou.net Cc: Jonathan Corbet corbet@lwn.net Cc: linux-doc@vger.kernel.org
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.
Documentation/iio/dmabuf_api.rst | 59 ++++++++++++++++++++++++++++++++ Documentation/iio/index.rst | 2 ++ 2 files changed, 61 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..4d70372c7ebd --- /dev/null +++ b/Documentation/iio/dmabuf_api.rst @@ -0,0 +1,59 @@ +.. 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.
+``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.
Rather than abusing subsections, this would be better done as a description list:
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.
Thanks,
jon