Landing within the Linux 3.15 kernel is initial NVIDIA Maxwell support from the kernel-side with Nouveau's Direct Rendering Manager. That support is still a first-stab at the support for this hardware that first began appearing in February in the form of the GeForce GTX 750 series. That Nouveau DRM code update for Maxwell is enough to light up the Maxwell GPUs with kernel-based mode-setting, but acceleration support is currently dependent upon the binary driver's microcode/firmware for proper hardware initialization.
On the user-space Gallium3D driver side, the work has been more of a challenge since Maxwell is a completely new instruction set architecture (ISA) compared to the Kepler GeForce GTX 600/700 series. After much work though, Ben Skeggs of Red Hat was able to push the initial code for Nouveau Maxwell into mainline Mesa.
The work that was pushed on Wednesday includes adding a new SM50 shader compiler back-end, enabling support for Maxwell boards, and other changes. The NVIDIA Maxwell support is built atop the Nouveau "NVC0" Gallium3D driver that supports the Fermi (GeForce 400/500) and Kepler (GeForce 600/700) series GPUs and now Kepler (GeForce GTX 750 and other upcoming products).
Soon as I get a chance to play with Linux 3.15 paired with this new Mesa 10.3-devel code and using the external firmware, I'll see how the support is working out. Sadly, like other NVIDIA GPUs on Nouveau, the biggest limitation will likely come down to its lack of re-clocking support for driving the hardware at maximum performance -- see Wednesday's Intel vs. Radeon vs. Nouveau On Linux 3.15 + Mesa 10.3-devel article for more details.