As illustrated in other articles, Hierarchical-Z can really boost graphics performance. Hierarchical Z (a.k.a. HiZ or also known as being part of the ATI/AMD HyperZ technology that's long been marketed) has already been implemented for the older open-source Radeon driver but hasn't been merged yet for any R600+ ASICs.
Hierarchical Z basically dumps unneeded pixels (as determined by checking them against the z-buffer, hence its name) as quickly as possible from the rendering pipeline. With less pixels to push forward, there's generally faster performance. Below are some very old ATI Technologies marketing slides concerning this feature.
While HyperZ / Hierarchical Z has been found within Radeon GPUs going back to the decade-old Radeon R100 series, it's open-source support hasn't been too quick to materialize. There's been HiZ support for pre-R600 (Radeon HD 2000 series), but the support for modern ATI/AMD graphics processors has been out of reach to most Linux desktop users.
The AMD Catalyst driver on Linux obviously supports Hyper Z's full capabilities, but the R600+ open-source support hasn't been mainlined. There's been R600+ Hierarchical Z patches floating in this FreeDesktop.org BugZilla bug for more than a year along with some patches in Alex Deucher's HTTP repository, but nothing that's worked its way mainline yet. Hierarchical Z support requires kernel DRM and Mesa (R600g) patches.
This afternoon updated Mesa/kernel patches arrived from Jerome Glisse in the aforelinked bug posting for open-source Hierarchical Z support on the GPUs from the Radeon HD 2000 series through the Radeon HD 6900 series. These patches were re-based to work atop the recent R600 tiling work.
The Hierarchical Z support for modern Radeon GPUs has been baking for about a year and we can only hope it's merged soon (assuming all issues are worked out) so that its performance can be more competitive with the proprietary Catalyst driver. This support would likely end up in the Linux 3.4 kernel and Mesa 8.1 releases.
What the Mesa patch specifically does is enable htile setup for R600g, which does a bit more than just HiZ. According to the patch description, "htile is used for HiZ and HiS support and fast Z/S clears. This commit just adds the htile setup. Fast Z/S clear is enabled. But we don't take full advantage of HiS with that patch." The kernel-side patch adds htile support to the Radeon DRM driver's command-stream (CS) checker.
The other patches found in Alex's directory for implementing the rest of the functionality in R600 Gallium3D don't appear to need any revisions at this time to work against the latest Mesa master code-base. The R600_SURF and R600_HYPERZ environment variables control the driver support.
Recent tests showed the open-source Radeon performance is about a half-decade behind Catalyst for many ATI/AMD GPUs, but the Hierarchical Z support does make the performance go up a fair amount. There's also the 2D tiling, PCI Express 2.0 support, and other not-yet-enabled-by-default features to up the frame-rates.
There will be some more Gallium3D benchmarks to publish in the near future.