mirror of
https://github.com/torvalds/linux.git
synced 2024-11-22 12:11:40 +00:00
d2f4831eaf
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>
105 lines
4.5 KiB
ReStructuredText
105 lines
4.5 KiB
ReStructuredText
====================
|
|
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
|
|
strings (the user space program has to provide the same information
|
|
that kernel level composite functions provide when they are added to
|
|
the configuration).
|
|
|
|
This in particular means that the composite initialisation functions
|
|
may not be in init section (ie. may not use the __init tag).
|
|
|
|
From user space point of view it is a file system which when
|
|
mounted provides an "ep0" file. User space driver need to
|
|
write descriptors and strings to that file. It does not need
|
|
to worry about endpoints, interfaces or strings numbers but
|
|
simply provide descriptors such as if the function was the
|
|
only one (endpoints and strings numbers starting from one and
|
|
interface numbers starting from zero). The FunctionFS changes
|
|
them as needed also handling situation when numbers differ in
|
|
different configurations.
|
|
|
|
When descriptors and strings are written "ep#" files appear
|
|
(one for each declared endpoint) which handle communication on
|
|
a single endpoint. Again, FunctionFS takes care of the real
|
|
numbers and changing of the configuration (which means that
|
|
"ep1" file may be really mapped to (say) endpoint 3 (and when
|
|
configuration changes to (say) endpoint 2)). "ep0" is used
|
|
for receiving events and handling setup requests.
|
|
|
|
When all files are closed the function disables itself.
|
|
|
|
What I also want to mention is that the FunctionFS is designed in such
|
|
a way that it is possible to mount it several times so in the end
|
|
a gadget could use several FunctionFS functions. The idea is that
|
|
each FunctionFS instance is identified by the device name used
|
|
when mounting.
|
|
|
|
One can imagine a gadget that has an Ethernet, MTP and HID interfaces
|
|
where the last two are implemented via FunctionFS. On user space
|
|
level it would look like this::
|
|
|
|
$ insmod g_ffs.ko idVendor=<ID> iSerialNumber=<string> functions=mtp,hid
|
|
$ mkdir /dev/ffs-mtp && mount -t functionfs mtp /dev/ffs-mtp
|
|
$ ( cd /dev/ffs-mtp && mtp-daemon ) &
|
|
$ mkdir /dev/ffs-hid && mount -t functionfs hid /dev/ffs-hid
|
|
$ ( cd /dev/ffs-hid && hid-daemon ) &
|
|
|
|
On kernel level the gadget checks ffs_data->dev_name to identify
|
|
whether its FunctionFS is designed for MTP ("mtp") or HID ("hid").
|
|
|
|
If no "functions" module parameters is supplied, the driver accepts
|
|
just one function with any name.
|
|
|
|
When "functions" module parameter is supplied, only functions
|
|
with listed names are accepted. In particular, if the "functions"
|
|
parameter's value is just a one-element list, then the behaviour
|
|
is similar to when there is no "functions" at all; however,
|
|
only a function with the specified name is accepted.
|
|
|
|
The gadget is registered only after all the declared function
|
|
filesystems have been mounted and USB descriptors of all functions
|
|
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.
|