FNA is an open source reimplementation of the Microsoft XNA Framework, specifically the 4.0 Refresh revision. It started as a branch of MonoGame called "MonoGame-SDL2".
The goal of the project is to recreate a fast, portable, and accurate XNA4 runtime library that prevents the need for code/content changes on the part of the XNA game developer. The currently supported platforms include Windows, Mac OS X and GNU/Linux, with further platform support including those supported by SDL2.
XNA, being a Microsoft technology, centered its controller input around the Xbox 360 Controller. Because of this, the XNA API is not cleanly compatible with custom controllers, including the Xbox 360 Controller when not being read by XInput. However, a config format was developed to allow binding of joystick input values to a virtual 360 controller, which then gets read by XNA games.
Since FNA uses SDL2 as its base library, we also get the benefit of SDL_GameController, a new subsystem added by Alfred Reynolds at Valve. This takes configurations for known controllers and automatically configures them for the virtual 360 controller, so if your controller is recognized, manual configuration is not necessary. If you run an FNA title through Steam, your controller will automatically be configured using the Big Picture Mode configuration.
In addition to expanded configuration support, FNA provides the GetGUIDEXT extension that allows XNA devs to pull in a trimmed/standardized GUID string to determine the controller hardware and react accordingly. TowerFall Ascension uses this for button icon support, for example.
Consider this GUID from the PlayStation 4 controller:
// These are the strings reported by SDL2's SDL_JoystickGetGUIDString:
4c05c405000000000000504944564944 // Windows. Note characters 1-8.
4c05000000000000c405000000000000 // OSX. Note characters 1-4 and 17-20.
030000004c050000c405000011010000 // Linux. Note characters 9-12 and 17-20.
4c05c405 // This is the GUID string returned by FNA. Get it?
For XInput controllers on Windows, the GUID string is simply 'xinput'.
FNA supports the DualShock 4 light bar, although it requires game developers to make use of the SetLightBarEXT extension to set the color of the light bar by way of adjusting the brightness of the RGB LEDs. Users also needs to allow write permission to brightness files per the below instructions before the game can make use of the light bar.
chmod 777 0003:054C:05C4.0021:red/brightness
chmod 777 0003:054C:05C4.0021:green/brightness
chmod 777 0003:054C:05C4.0021:blue/brightness