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==============
USB Raw Gadget
==============
USB Raw Gadget is a kernel module that provides a userspace interface for
the USB Gadget subsystem. Essentially it allows to emulate USB devices
from userspace. Enabled with CONFIG_USB_RAW_GADGET. Raw Gadget is
currently a strictly debugging feature and shouldn't be used in
production, use GadgetFS instead.
Comparison to GadgetFS
~~~~~~~~~~~~~~~~~~~~~~
Raw Gadget is similar to GadgetFS, but provides a more low-level and
direct access to the USB Gadget layer for the userspace. The key
differences are:
1. Every USB request is passed to the userspace to get a response, while
GadgetFS responds to some USB requests internally based on the provided
descriptors. However note, that the UDC driver might respond to some
requests on its own and never forward them to the Gadget layer.
2. GadgetFS performs some sanity checks on the provided USB descriptors,
while Raw Gadget allows you to provide arbitrary data as responses to
USB requests.
3. Raw Gadget provides a way to select a UDC device/driver to bind to,
while GadgetFS currently binds to the first available UDC.
4. Raw Gadget uses predictable endpoint names (handles) across different
UDCs (as long as UDCs have enough endpoints of each required transfer
type).
5. Raw Gadget has ioctl-based interface instead of a filesystem-based one.
Userspace interface
~~~~~~~~~~~~~~~~~~~
To create a Raw Gadget instance open /dev/raw-gadget. Multiple raw-gadget
instances (bound to different UDCs) can be used at the same time. The
interaction with the opened file happens through the ioctl() calls, see
comments in include/uapi/linux/usb/raw_gadget.h for details.
The typical usage of Raw Gadget looks like:
1. Open Raw Gadget instance via /dev/raw-gadget.
2. Initialize the instance via USB_RAW_IOCTL_INIT.
3. Launch the instance with USB_RAW_IOCTL_RUN.
4. In a loop issue USB_RAW_IOCTL_EVENT_FETCH calls to receive events from
Raw Gadget and react to those depending on what kind of USB device
needs to be emulated.
Potential future improvements
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- Implement ioctl's for setting/clearing halt status on endpoints.
- Reporting more events (suspend, resume, etc.) through
USB_RAW_IOCTL_EVENT_FETCH.
- Support O_NONBLOCK I/O.
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