[shazow]

The API is open, the SteamVR-specific runtime is a binary blob.

Anyone can implement a driver, they have example driver code available. The documentation is here: https://github.com/ValveSoftware/openvr/wiki

The purpose of OpenVR is to create a common API for HMD drivers, so that other platforms can implement their own proprietary equivalents of SteamVR or what have you, and everyone's HMD drivers will be interoperable.