Andres Rodriguez working for Valve as part of their Linux graphics driver team has begun landing his work around OpenGL semaphores for Mesa/Gallium3D and following through with the necessary bits for RadeonSI with the EXT_semaphore work being needed by SteamVR…

Source: Phoronix – RadeonSI Semaphore Begins Hitting Mesa Git, Needed By SteamVR On Linux