1==============
2USB Raw Gadget
3==============
4
5USB Raw Gadget is a kernel module that provides a userspace interface for
6the USB Gadget subsystem. Essentially it allows to emulate USB devices
7from userspace. Enabled with CONFIG_USB_RAW_GADGET. Raw Gadget is
8currently a strictly debugging feature and shouldn't be used in
9production, use GadgetFS instead.
10
11Comparison to GadgetFS
12~~~~~~~~~~~~~~~~~~~~~~
13
14Raw Gadget is similar to GadgetFS, but provides a more low-level and
15direct access to the USB Gadget layer for the userspace. The key
16differences are:
17
181. Every USB request is passed to the userspace to get a response, while
19   GadgetFS responds to some USB requests internally based on the provided
20   descriptors. However note, that the UDC driver might respond to some
21   requests on its own and never forward them to the Gadget layer.
22
232. GadgetFS performs some sanity checks on the provided USB descriptors,
24   while Raw Gadget allows you to provide arbitrary data as responses to
25   USB requests.
26
273. Raw Gadget provides a way to select a UDC device/driver to bind to,
28   while GadgetFS currently binds to the first available UDC.
29
304. Raw Gadget uses predictable endpoint names (handles) across different
31   UDCs (as long as UDCs have enough endpoints of each required transfer
32   type).
33
345. Raw Gadget has ioctl-based interface instead of a filesystem-based one.
35
36Userspace interface
37~~~~~~~~~~~~~~~~~~~
38
39To create a Raw Gadget instance open /dev/raw-gadget. Multiple raw-gadget
40instances (bound to different UDCs) can be used at the same time. The
41interaction with the opened file happens through the ioctl() calls, see
42comments in include/uapi/linux/usb/raw_gadget.h for details.
43
44The typical usage of Raw Gadget looks like:
45
461. Open Raw Gadget instance via /dev/raw-gadget.
472. Initialize the instance via USB_RAW_IOCTL_INIT.
483. Launch the instance with USB_RAW_IOCTL_RUN.
494. In a loop issue USB_RAW_IOCTL_EVENT_FETCH calls to receive events from
50   Raw Gadget and react to those depending on what kind of USB device
51   needs to be emulated.
52
53Potential future improvements
54~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
55
56- Implement ioctl's for setting/clearing halt status on endpoints.
57
58- Reporting more events (suspend, resume, etc.) through
59  USB_RAW_IOCTL_EVENT_FETCH.
60
61- Support O_NONBLOCK I/O.
62