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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>
This commit is contained in:
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
====================
Overview
========
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
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
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.