Just yesterday morning is when the GeForce GTX 900 signed firmware support was published after these open-source community developers waited the better part of two years. This was a big milestone albeit long overdue.
By the time I went to sleep, the lead Nouveau developer already got 3D games running on the open-source driver stack and making nice progress with not being blocked by the firmware obstacle.
Waking up this morning, Ben Skeggs already went ahead and landed the initial GeForce GTX 900 series (GM20x) support in Mesa's NVC0 Gallium3D driver. There is now the initial support for this latest generation of GPUs. There is also the Maxwell texture headers, which NVIDIA provided to the community back in December.
It's great to see the initial GTX 900 series support land in Mesa so quickly considering the Mesa 11.2 branching is set to happen on Friday, which now to much surprise will have these basic user-space bits. Keep in mind though that this won't be workable until either building your own kernel from the Nouveau Git branch or waiting for the Linux 4.6 kernel to enter its development stages. You'll also need to grab the NVIDIA-signed firmware blobs. There's also likely some initial early pains such as my recently outlined GTX 750 Nouveau woes that are still outstanding -- one of the main blockers to usability being the lack of re-clocking support.