It will not be officially rolled in until kernel 4.2 but you can currently grab the new binary blob by following the links from Phoronix.  This new AMDGPU kernel driver will be used by both the full open-source driver and the Catalyst driver provided officially by AMD and provide support not only for the R9 285 but upcoming families as well.  There is still some development to be done as AMD's Alex Deucher told Phoronix that this initial code lacks power management features for Tonga but that will be addressed shortly.

"At long last the source code to the new AMDGPU driver has been released! This is the new driver needed to support the Radeon R9 285 graphics card along with future GPUs/APUs like Carrizo. Compared to the existing Radeon DRM driver, the new AMDGPU code is needed for AMD's new unified Linux driver strategy whereby the new Catalyst driver will be isolated to being a user-space binary blob with both the full open-source driver and the Catalyst driver using this common AMDGPU kernel driver."

Here is some more Tech News from around the web:

Tech Talk