NVIDIA's Alexandre Courbot went ahead and committed to linux-firmware.git the GM20B signed firmware as needed by the Tegra X1 SoC. There's also now the GM200, GM204, and GM206 signed firmware.
The kernel-side Nouveau changes for supporting the signed firmware is expected to land for the Linux 4.6 kernel. In user-space, Mesa work is hitting Mesa Git for the latest Maxwell GPU support. However, re-clocking and other missing functionality makes the current open-source GTX 900 series still less than ideal for Linux desktop end-users.