We've long been monitoring the AMD Hawaii Linux support situation. AMD did provide same-day Catalyst Linux support for the R9 290 but took a while to get cleaned up. However, on the open-source side, it wasn't until recently that the R9 290 open-source support got into shape with working 2D/3D hardware acceleration.
Hawaii GPU acceleration should be working with the latest open-source code but users have reported that re-clocking isn't being done properly yet (at least not by default) and there still may be some other issues to work out along with overall optimizations still being sought after with the RadeonSI Gallium3D driver.
Getting R9 290 "Hawaii" series hardware to work requires using the newest DRM found in the Linux 3.17 kernel. Additionally, the updated Hawaii microcode files need to be present for the acceleration to work. The latest microcode/firmware files can be downloaded from this directory. Additionally, for good measure you should also be running the very latest Mesa Git code for the best RadeonSI support (currently at Mesa 10.3-devel). Up to now users of the high-end AMD hardware have also needed to set the Accel option in the xorg.conf to true for enabling the hardware acceleration when meeting the other requirements.
As of this Git commit today, Hawaii acceleration support is being enabled by default with xf86-video-ati assuming the kernel reports the updated DRM/microcode state. This default change will be present in the upcoming xf86-video-ati 7.5.0 release.
Now that the Linux 3.17 merge window is settling down with the Hawaii support, I'll be out with Radeon R9 290 Linux benchmarks of the open-source driver compared to Catalyst in the near future.