Since the Hawaii GPUs began shipping I have been running many tests of a Radeon R9 290 graphics card that I purchased. When using the Catalyst proprietary driver, the R9 290 tends to work well but its performance is poor with Catalyst when compared to the latest-generation NVIDIA hardware with their proprietary Linux graphics driver. With the open-source driver, the R9 290 and R9 290X still don't work right.
Last month I wrote about Trying Out Radeon R9 290 Graphics On Open-Source and since then things still don't work right even as Mesa 10.2 is soon being released along with the Linux 3.15 kernel. During my last attempts at the open-source Hawaii support, I ran into show-stopping issues with GPU faults and wasn't able to get the R9 290 playing at all with the open-source driver unless sticking to having the hardware acceleration disabled.
Now there is FreeDesktop.org Bug #78453, "[HAWAII] Get acceleration working." The bug report by Luzipher reads, "Hawaii acceleration is currently disabled by default, as it doesn't really work (causes GPU crashes). This bug intends to collect data that hopefully helps to resolve the issues. [Alex Deucher] suggested on irc that, when hawaii support was first committed, it worked better than now. Back then, glxgears worked according to his words. He also said it'd help to know which component caused the regression for glxgears. I tried to get glxgears working with software versions from back then, but I wasn't successful."
So when the AMD Hawaii support first landed, it was good enough for running glxgears but now it doesn't even work that well -- the same as my experiences. Alex posted a patch last night that might help for Hawaii's lack of linear depth/stencil buffers, but the original poster hasn't yet tested it nor do the open-source AMD developers seem particularly concerned (albeit everyone has time commitments) about the Hawaii support -- hence letting the support get stale for AMD's current highest-end hardware. Soon as we have the Hawaii R9 290 running on the RadeonSI Gallium3D driver, you can expect to see a plethora of benchmarks on Phoronix to see how it compares to the AMD Catalyst Linux driver.