The Lenovo Legion Go is a handheld gaming system, similar to a Steam Deck.
It has a gamepad (including rear paddles), 3 gyroscopes, a trackpad,
volume buttons, a power button, and 2 LED ring lights.

The Legion Go firmware presents these controls as a USB hub with various
devices attached.  In its default state, the gamepad is presented as an
Xbox controller connected to this hub.  (By holding a combination of
buttons, it can be changed to use the older DirectInput API.)

This patch teaches the existing Xbox controller module `xpad` to bind to
the controller in the Legion Go, which enables support for the:

- directional pad,
- analog sticks (including clicks),
- X, Y, A, B,
- start and select (or menu and capture),
- shoulder buttons, and
- rumble.

The trackpad, touchscreen, volume controls, and power button are already
supported via existing kernel modules.  Two of the face buttons, the
gyroscopes, rear paddles, and LEDs are not.

After this patch lands, the Legion Go will be mostly functional in Linux,
out-of-the-box.  The various components of the USB hub can be synthesized
into a single logical controller (including the additional buttons) in
userspace with [Handheld Daemon](https://github.com/hhd-dev/hhd), which
makes the Go fully functional.
Signed-off-by: Brenton Simpson <appsforartists@google.com>
Link: https://lore.kernel.org/r/20240118183546.418064-1-appsforartists@google.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>