Published yesterday were the Vega AMDGPU kernel patches and RadeonSI Gallium3D enablement while the Vega changes to its LLVM back-end are ongoing. This evening from Marek Olšák is the libdrm changes.
These are the libdrm changes needed for supporting "Vega 10". Libdrm itself isn't too exciting, but Marek mentioned in the patch series, "I do plan to push
these before the kernel support lands (including the amdgpu_drm.h change), so that I can push the Mesa support."
While generally they wait to push libdrm/Mesa support until the kernel-side changes are made, since the support isn't useful/possible without the DRM/KMS driver support and in case any of the kernel interfaces were to change prior to merging. But that would mean they wouldn't be able to merge until after the Linux 4.12 merge window in late April / early May (assuming the DC prerequisite and AMDGPU Vega changes make it in), but the Mesa 17.1 branching is set to happen mid-April. So now it looks like AMD will try to push their libdrm/Mesa support early, presumably in time for Mesa 17.1 if all the necessary code reviews happen on the 140+ new patches.
Mesa 17.1 will then be released as stable in May, but for any early Vega10 owners, it won't do much good without also having the supported kernel code as well as LLVM 5.0 SVN/Git. At least though it looks like they're doing their best to get this work mainlined ASAP to ease the open-source experience for Linux enthusiasts/gamers planning to purchase these new high-end AMD graphics cards expected around late May. Those not wanting to run bleeding-edge kernels, LLVM, and Mesa, AMD should at least have out a same-day AMDGPU-PRO driver with Vega Linux support on their hybrid stack.