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Documentation: usb: Document FunctionFS DMABUF API 

Add documentation for the three ioctls used to attach or detach
externally-created DMABUFs, and to request transfers from/to previously
attached DMABUFs.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20240130122340.54813-5-paul@crapouillou.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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Paul Cercueil 2024-01-30 13:23:40 +01:00 committed by Greg Kroah-Hartman
parent 7b07a2a7ca
commit d2f4831eaf
1 changed files with 36 additions and 0 deletions
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36
Documentation/usb/functionfs.rst
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@ -2,6 +2,9 @@
How FunctionFS works How FunctionFS works
==================== ====================
Overview
========
From kernel point of view it is just a composite function with some From kernel point of view it is just a composite function with some
unique behaviour. It may be added to an USB configuration only after unique behaviour. It may be added to an USB configuration only after
the user space driver has registered by writing descriptors and the user space driver has registered by writing descriptors and
@ -66,3 +69,36 @@ have been written to their ep0's.
Conversely, the gadget is unregistered after the first USB function Conversely, the gadget is unregistered after the first USB function
closes its endpoints. closes its endpoints.
DMABUF interface
================
FunctionFS additionally supports a DMABUF based interface, where the
userspace can attach DMABUF objects (externally created) to an endpoint,
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.
As part of this interface, three new IOCTLs have been added. These three
IOCTLs have to be performed on a data endpoint (ie. not ep0). They are:
``FUNCTIONFS_DMABUF_ATTACH(int)``
Attach the DMABUF object, identified by its file descriptor, to the
data endpoint. Returns zero on success, and a negative errno value
on error.
``FUNCTIONFS_DMABUF_DETACH(int)``
Detach the given DMABUF object, identified by its file descriptor,
from the data endpoint. Returns zero on success, and a negative
errno value on error. Note that closing the endpoint's file
descriptor will automatically detach all attached DMABUFs.
``FUNCTIONFS_DMABUF_TRANSFER(struct usb_ffs_dmabuf_transfer_req *)``
Enqueue the previously attached DMABUF to the transfer queue.
The argument is a structure that packs the DMABUF's file descriptor,
the size in bytes to transfer (which should generally correspond to
the size of the DMABUF), and a 'flags' field which is unused
for now. Returns zero on success, and a negative errno value on
error.