Following the recent article about Valve & Intel Work On Open-Source GPU Drivers, hitting the mailing list for Mesa development today was a set of patches by Intel's Eric Anholt with changes at the request of Valve.
The patch-set comes down to exposing more information than what is currently exposed through the GL_ARB_debug_output extension. Some of the information now exposed through the performance debug information include messages for freagment and vertex shaders triggering register spilling, GPU stalls waiting for a query object, advice to using a blit call for avoiding a stall on glBufferSubData(), stalling the GPU within glBufferSubData(), mapping a busy buffer object, failing to fast clear depth, notice on recompiling shaders, the state cache size limit being exceeded, and notice for stalls during shader compilation.
These performance debugging messages can benefit any OpenGL game/application developer, but are just available at this point for the i965 DRI driver and does require setting INTEL_DEBUG=perf.
One of Valve's requests was for GL_ARB_debug_output for performance traps they should know about. Unfortunately, Mesa's ARB_debug_output support is very limited at the moment, so this just gets messages in place, which we can convert to GL_ARB_debug_output at some later time.