With VDrift, the open-source Gallium3D stack was much faster than the proprietary drivers. However, this is nothing that is Evergreen or Northern Islands / Cayman specific. Older Radeon graphics cards on Mesa / Gallium3D as well as NVIDIA hardware on the Nouveau Gallium3D driver is still faster than the respective binary driver. This performance "advantage" appears to be attributed to Mesa not correctly rendering all textures correctly.
Aside from Lightsmark where the Radeon HD 6000 series hardware dropped to about 30% the speed of the proprietary Catalyst driver, in the other tests the very latest Linux 3.0 kernel and Mesa 7.12-devel Git code was running at around 50~60% the speed of the Catalyst driver with ease. In some tests, the R600 Gallium3D driver on Northern Islands / Cayman was even 70~80% for these older OpenGL games that are capable of running on Mesa without its OpenGL 3/4 support and lack of S3TC support by default, etc. The open-source driver also had the non-default options of disabling swap buffers wait and enabling color tiling, which does measurably boost the open-source driver performance. These results from our initial Radeon HD 6000 Gallium3D benchmarking can be fetched from OpenBenchmarking.org.
It is nice to see the Radeon HD 6000 series hardware performing rather favorably compared to the Catalyst driver, but this is coming only after the support was broken for several months. The first Northern Islands hardware made it out back in October of 2010, and only after nine months, is there a satisfiable level of working open-source support. This though is what is found in Git currently, so unless you intend to be pulling in the latest Linux 3.0 kernel and Mesa 7.11/7.12-devel code, you are best off waiting on a HD 6000 series GPU until your distribution of choice provides updated packages. For Ubuntu users, this means October with the release of Ubuntu 11.10. The Radeon HD 4000/5000 series has stable open-source support right now, but with the Git code, the Radeon HD 6000 series is beginning to match.
Of course, there is also still no Unified Video Decoder support in the open-source driver, less than desirable power management and other shortcomings of going with the open-source driver, but that is nothing Radeon HD 6000 series specific. There is also the sad OpenGL limitations of Mesa. Regardless, moving forward we will hopefully see more timely open-source hardware support from AMD now that AMD hired two more open-source Linux developers and they are aiming for some fundamental changes by the Radeon HD 8000 series.
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