Valve has granted these Intel Linux developers complete access to the game's source-code, including the Source Engine. This has allowed Intel's Linux developers to better investigate possible optimizations and tweaks to their driver in order to enhance Source-powered games. Valve has even given them commit access to push back changes to the game company.
Intel developers are able to push changes into a Perforce tree that Valve uses. With the source code access, Intel developers fixed context creation, a texture sampling bug, and other issues for Valve's closed-source game. Having complete source access, Eric describes this Valve Linux experience as being the best video game debugging interation ever.
While these Intel developers are happy looking at the Source Engine code, Valve Linux developers have also been happy looking at an open-source graphics driver. Valve Linux developers found it equally thrilling that now when hitting a bottleneck in their game or looking for areas for performance optimizations, they are simply able to look into Intel's open-source Linux graphics driver to understand how an operation is handled by the hardware, tossing some extra debugging statements into the Intel driver to see what's happening, and making other driver tweaks.
As shared in the Valve L4D2 presentation slides from their OpenGL SIGGRAPH session in August, the Source Engine is using some abstraction with an API similar to Direct3D for ultimately targeting OpenGL. Eric added during the session that Valve is currently targeting OpenGL 3.2 with its core API and not the compatibility API.
Anholt admitted that Valve has reached performance parity on another driver (the NVIDIA Linux blob -- see How Valve Made L4D2 Faster On Linux Than Windows) but they haven't yet hit a performance parity with the Intel graphics driver on Linux. This though isn't exactly a surprise, since the Intel Linux graphics driver is generally much slower than the Intel Windows driver, as illustrated this morning in the OS X vs. Windows vs. Linux benchmarks.
Besides not yet being at a performance parity, there's also one rendering bug left with the Intel Mesa driver -- there's some blue textures in Left 4 Dead 2.