Thierry Reding has been the one independently working on this NVIDIA Tegra DRM driver that adds support for the "host1x" and a sub-set of the display controller hardware found on NVIDIA Tegra SoCs. The current state allows for RGB/LVDS and HDMI outputs and does leverage the GEM/CMA and KMS/FB work. Right now though this Tegra DRM driver doesn't support any 3D bits.
Interestingly, some people from NVIDIA have even helped out with this open-source kernel graphics driver. "During the development of this series I've received a lot of feedback and many helpful suggestions from the people at NVIDIA, so I owe them big thanks."
The Tegra DRM driver in its current form is about 4,500 lines of new code. The most recent version of this driver was published on Friday and is floating on the dri-devel list.
Meanwhile, we are still waiting for NVIDIA to release some Tegra programming documentation as they promised to do back during XDC2012.