How do steering wheels work as input/output devices?

Question

I am trying to understand problems with how steering wheels work on Linux (or not). Does every model of steering wheel require a specific driver? And why? I think that they shouldn't need special drivers because all wheels are very similar as far as I know (send the angle of the wheel and position of pedals and buttons and receive force feedback), so one driver could work with all wheels like one driver works with all USB keyboards or most sound devices. But I see projects making specific models of wheels work on Linux (like hid-fanatecff), which hints to that they need special drivers.

That gives me the idea: How do games communicate with the steering wheels? Do they need special support for specific wheels? Keeping support for all steering wheels seems unmanageable, so I guess that there is a common interface, but I couldn't find one.

Answer 1

For inputs (wheel axis, pedal axes, buttons) you generally do not need a specific driver. Steering wheel controllers are HID devices and typically assign standard HID usage values to the report fields for these inputs. This allows the default HID input driver to automatically convert the inputs to Linux input events.

The reason why wheel controllers need drivers has more to do with lack of standardization for force feedback and other advanced features. There are standard HID usage values for force feedback but they're relatively recent additions and it's not as common to see them used in wheel controllers. This means force feedback support typically requires vendor-specific logic, and sometimes model-specific logic.

There may be other special features that need vendor-specific logic to support. For example, Logitech racing wheels are "multimode" which means they have compatibility modes to allow newer models to behave like older models. When the device is first connected it defaults to a basic compatibility mode. The driver detects the wheel's actual model and initializes the wheel into the most advanced mode supported by the wheel and the driver. Switching to the advanced mode may enable additional features, like the ability to increase the range of the wheel or use more advanced force feedback effects.