The XA state tracker was developed by VMware as a way of providing 2D acceleration using OpenGL (similar in nature to GLAMOR but implemented in Gallium3D) for Gallium drivers. XA is used by the VMware SVGA 2 graphics driver but hasn't seen too much adoption outside of the virtualization company. Rob is now turning to XA for faster 2D performance within this reverse-engineered driver.
In the patches enabling Xa for Freedreno, the xf86-video-freedreno DDX is experiencing around a 2x performance boost for es2gears, glmark2, and other performance benchmarks. The patch also makes changes to the XA state tracker so it can be used by more than VMware's graphics DDX.
Those interested in more details can find the queued up patches on the Mesa mailing list and should land with plenty of time for Mesa 10.2.