A message hit the Mesa development mailing list today entitled r600g: hyperz, from veteran open-source ATI/AMD contributor Jerome Glisse of Red Hat. He's posted a new R600g HyperZ patch, but it looks like it might be his last.
Jerome says he's been working on the R600g open-source HyperZ enablement for the past seven monthsm but he fails at not making it lock-up. He can't figure out why HyperZ is causing GPU lock-ups on the open-source driver, but he's trying to push it upstream anyhow to avoid rebasing the work constantly.
The French developer went on to say that he would try to match the fglrx (Catalyst) Linux driver's sync and flush pattern, but that would basically mean rewriting the entire r600g driver. He tried just cleaning up the R600g flushing, but that still doesn't avoid lock-ups.
Jerome ends his email with, "So if the feature doesn't lockup you will see on average 5% increase in performance. Anyway off to work on something else."
At the same time, Jerome also released an updated HTILE R600g patch (the mailing list message). The HTILE support patch for R600g is used for the HiZ and HiS support with fast Z/S clears.
For more information on the matter, see Radeon HyperZ In Open-Source On Older Hardware, AMD R600g Still Tackling Hierarchical Z, and An Optimized Open-Source Driver Tries To Compete With AMD Catalyst.