For the xf86-video-ati X.Org driver there is already mainline in Git AMD Hawaii support along with the initial PCI IDs. The support went straight away into xf86-video-ati Git since the changes are minimal. The hardware acceleration though is disabled by default until the RadeonSI 3D support for the Hawaii GPUs matures.
Like the Radeon HD 7000/8000 series GPUs, the Hawaii GPUs are reliant upon GLAMOR for having 2D EXA acceleration implemented via OpenGL. Thus in order to have good 2D support, good 3D support is a must. Until the 3D support is ironed out for Hawaii -- and it's still maturing for the first-generation GCN hardware -- it's being disabled by default. Those wanting to use experimental 2D/3D support when running all of the latest code will for now need to set NoAccel to false in the xorg.conf.
Next up, the libdrm code-base received its straightforward work today in supporting Hawaii GPUs and the first batch of PCI IDs. Again the initial AMD Hawaii PCI IDs are 0x67A0, 0x67A1, 0x67A2, 0x67A8, 0x67A9, 0x67AA, 0x67B0, 0x67B1, 0x67B8, 0x67B9, 0x67BA, and 0x67BE.
To the AMD GPU R600 LLVM back-end in upstream LLVM there needs to be a simple patch that sets the Hawaii processor type to the same as the Sea Islands GPU support. Hopefully this patch will make it into next month's LLVM 3.4 release.
The Hawaii support for AMD's RadeonSI Gallium3D driver isn't living in Mesa Git master but they are floating on the mailing list pending review. There is one patch that changes around just a few lines of code for supporting Hawaii ASICs and updating some additional register fields followed by another patch that just adds in the new PCI IDs.
So if you want AMD Hawaii open-source GPU driver support it's looking like you will need the Linux 3.13 kernel, xf86-video-ati 7.3.0, libdrm 2.4.48, LLVM 3.4, GLAMOR, and Mesa 10.0 (assuming the small patches get accepted into the branch code). As said earlier, 2D/3D acceleration support is disabled by default for now so you'll need to set the X configuration option too. Overall you should be sticking to all of the latest Git code if planning to use one of the new high-end GPUs or just sticking with the binary Catalyst driver for a couple of months until the RadeonSI support is more up to par.
For Radeon R9 290 Linux benchmarks see the Phoronix article earlier today.