[PATCH v5 6/6] Documentation: usb: Document FunctionFS DMABUF API

Paul Cercueil
2024-01-19T14:15:03Z

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>

---
v3: New patch
---
Documentation/usb/functionfs.rst | 36 ++++++++++++++++++++++++++++++++
1 file changed, 36 insertions(+)

diff --git a/Documentation/usb/functionfs.rst b/Documentation/usb/functionfs.rst
index a3054bea38f3..d05a775bc45b 100644
--- a/Documentation/usb/functionfs.rst
+++ b/Documentation/usb/functionfs.rst
@@ -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.
--
2.43.0