The RadeonSI driver supports the newer Radeon HD 7000/8000 series hardware and is less mature than the R600 Gallium3D driver. The RadeonSI driver right now only advertises OpenGL 2.1 support, the performance is less than R600g, and there's other limitations, albeit AMD has been devoting resources to improving this driver given it's used by the latest-generation AMD products.
Released on Wednesday were a set of six patches by Marek to provide minor clean-ups and improvements to this open-source Radeon Gallium3D driver. The six patches touch around 2,500 lines of code. More interestingly though these patches are a prerequisite to the more interesting OpenGL Transform Feedback support that Marek has been working on for RadeonSI.
So far the review of these six RadeonSI patches have been good by fellow open-source AMD developer so likely this work will be merged to Mesa Git master soon, thus allowing for more RadeonSI advancements to happen. Aside from this work by Marek, he's also made old R300/R400 driver changes this week too.