the mainline kernel works thanks to the work of volunteers
Google also bankrolls a lot of effort around the Graphics stack in support of ChromeOS (Chromebooks). Most of the work done by Collabora has its roots in ChromeOS needs. Rockchip staff are also upstreaming a lot of the core drivers and board support. There is still a lot of work done by volunteers, but as vendors go, Rockchip aren't bad (better than most, worse than others).