It looks like there will be open-source Linux driver support from launch-day for AMD's forthcoming "Richland" Fusion APUs.
AMD's Richland APU is expected to be released in the next month or two and is a revised/updated version of last year's Trinity APUs. Richland is to be manufactured on a 32nm process with dual or quad Piledriver CPU cores with AMD Radeon HD 8000 series graphics. Richland is basically an improved version of Trinity.
With this being integrated APU graphics and not some new Radeon HD GPU design, adding the support isn't a big deal. In fact, it's not even to the still-being-developed "RadeonSI" Gallium3D driver but is for the more reliable and stable "R600g" driver.
The Richland PCI IDs were added today and the code for the open-source driver reflects them as being ARUBA, the same as the original Trinity APUs. The PCI IDs added for the Fusion Richland APUs to the open-source driver are 0x990B, 0x990C, 0x990D, 0x990E, 0x995, 0x9996, 0x9997, 0x9998, 0x9999, 0x999A, and 0x999B.
While there's these open-source driver patches pre-launch for the xf86-video-ati DDX, libdrm, Mesa, and the Radeon DRM kernel driver, unfortunately, it won't be "out of the box" same-day support. These Git commits come too late where by next month they won't be found in released packages found in the common Linux distributions. Being based upon the original Trinity design and just needing the new PCI IDs, they can be back-ported to stable branches, but unless you're running a rolling-release distribution chances are they won't be readily available to you in an easy manner. You'll need to use the AMD Catalyst driver or likely be building the necessary driver components yourself from source.
Here's the Mesa commit
for supporting the AMD Richland APUs, among the other Git repositories needed where the Richland support was also pushed this morning by AMD's Alex Deucher.