usb: gadget: add raw-gadget interface

    misc_usbsevseg

    mtouchusb

    ohci

    raw-gadget

    usbip_protocol

    usbmon

    usb-serial

==============

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.

	  Say "y" to link the driver statically, or "m" to build a

	  dynamically linked module called "g_webcam".

config USB_RAW_GADGET

	tristate "USB Raw Gadget"

	help

	  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. See Documentation/usb/raw-gadget.rst for

	  details.

	  Say "y" to link the driver statically, or "m" to build a

	  dynamically linked module called "raw_gadget".

endchoice

obj-$(CONFIG_USB_G_NCM)		+= g_ncm.o

obj-$(CONFIG_USB_G_ACM_MS)	+= g_acm_ms.o

obj-$(CONFIG_USB_GADGET_TARGET)	+= tcm_usb_gadget.o

obj-$(CONFIG_USB_RAW_GADGET)	+= raw_gadget.o