Gamepad support in Sommelier

When GAMEPAD_SUPPORT is enabled, Sommelier will create a gaming seat to listen for gamepads being attached and their associated events.

Once a gamepad is added, we use evdev and uinput to emulate an Xbox One S gamepad in the VM. We subsequently forward gamepad events from the host through that emulated gamepad. Because not every gamepad lines up precisely with an Xbox gamepad, we sometimes have to remap input events so that they make sense.

Adding new gamepad mappings

This guide assumes you have access to a Chromebook in Developer Mode.

1. First, you‘ll need the product, vendor and version ID of the gamepad you’re adding. You’ll also need the input event code values that correspond with each input on your gamepad.
   • We suggest using evtest to gather this information
     1. Open a shell on your DUT and connect your gamepad
     2. Run evtest and select your device
     3. Copy the ID values from the “Input device ID” section
     4. Press/use each input on your device and note what input event code values evtest observes
2. Add your mapping to sommelier-gaming
   1. Create a DeviceID struct with the details of your gamepad
   2. Create (or reuse) a mapping for your gamepad
   3. Add the DeviceID/mapping pair to kDeviceMappings

To figure out what the mapping should be, you can follow the reference table below which demonstrates which event codes are mapped to which buttons on an Xbox gamepad (which we emulate) - we suggest making the mappings locality focused, rather than glyph-focused (i.e bottom face button should map to the Xbox “A”).

Xbox One S gamepad button-event code mapping:

Button/axes	Input code
left joystick	ABS_X, ABX_Y
right joystick.	ABS_RX, ABS_RY
directional pad	ABS_HAT0X, ABS_HAT0Y
left bumper	BTN_TL
left trigger	ABS_Z
right bumper	BTN_TR
right trigger	ABS_RZ
Y	BTN_Y
B	BTN_B
A	BTN_A
X	BTN_X
select button	BTN_SELECT
Xbox button	BTN_MODE
start button	BTN_START

Notes:

• If your gamepad is missing axes or buttons, you may need to do additional work to support them
  • e.g for the DS3/PS3 gamepad we needed to add additional logic to convert BTN_DPAD_* buttons into ABS_HAT* axes and “activate” those axes manually.
• You shouldn’t need to add any additional unit tests - unless you’re adding a non-conventional gamepad (as described above).
• You’ll need to follow these steps for each connection type you want to support (i.e you’ll need to add support for USB and Bluetooth separately).
• In some cases kernel versions, driver updates, and gamepad firmware updates may change the behaviour of a gamepad. Firmware updates would typically change the version ID, however.