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>

Author: pcercuei

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.