docs: usb: Add documentation for the UVC Gadget

The UVC Gadget function has become quite complex, but documentation
for it is fairly sparse. Add some more detailed documentation to
improve the situation.

Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Link: https://lore.kernel.org/r/20230308165213.139315-1-dan.scally@ideasonboard.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

usb: gadget: add raw-gadget interface

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.

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.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>

docs: usb: remove some broken references

It seems that some files were removed from USB documentation.

Update the links accordingly.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/00008303fde6b4e06d027d3b76ae7032614a7030.1580193653.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

docs: usb: rename files to .rst and add them to drivers-api

While there are a mix of things here, most of the stuff
were written from Kernel developer's PoV. So, add them to
the driver-api book.

A follow up for this patch would be to move documents from
there that are specific to sysadmins, adding them to the
admin-guide.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Johan Hovold <johan@kernel.org>
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>