Leo posted a set of patches that add more H.264 AVC profiles to the Gallium3D video layer, implements H.264 profile support for the OpenMAX state tracker's encoder, adds H.264 profile support to the Radeon VCE code, and enables B frame support for OpenMAX encoding. Long story short, these patches allow Radeon H.264 video encoding on the open-source Linux graphics driver when using a modern GCN card that supports "VCE 2.0" for video encoding.
AMD's small open-source crew has been the principal developers behind the OpenMAX state tracker, which premieres with Mesa 10.2. With there already being the VDPAU state tracker that provides great video decoding support on the open-source Radeon driver with UVD-supported hardware, the primary purpose of this OpenMAX support has been for exposing AMD's video encoding acceleration on recent GPUs.
A few months ago is when AMD open-sourced their VCE 2.0 video encode support with the Mesa/Gallium3D bits and the Radeon DRM kernel driver -- found in Linux 3.15.
These H.264 profile support patches for the OpenMAX and Radeon code should hopefully land well in time for the Mesa 10.3 release in about three months time.