Axel Davy's work for DRI3 offloading in Mesa hit Git last night and he explains with the commit for this feature to use secondary GPUs for rendering:
The differences with DRI2 GPU offloading are:
a) There's no logic for GPU offloading needed in the Xserver
b) for DRI2, the card would render to a back buffer, and the content would be copied to the front buffer (the same buffers everytime). Here we can potentially use several back buffers and copy to buffers with no tiling to share with X. We send them with the Present extension.
That means than the DRI2 solution is forced to have tearings with GPU offloading. In the ideal scenario, this DRI3 solution doesn't have this problem.
However without dma-buf fences, a race can appear (if the card is slow and the rendering hasn't finished before the server card reads the buffer), and then old content is displayed. If a user hits this, he should probably revert to the DRI2 solution (LIBGL_DRI3_DISABLE). Users with cards fast enough seem to not hit this in practice (I have an Amd hd 7730m, and I don't hit this, except if I force a low dpm mode)
c) for non-fullscreen apps, the DRI2 GPU offloading solution requires compositing. This DRI3 solution doesn't have this requirement. Rendering to a pixmap also works.
d) There is no need to have a DDX loaded for the secondary card.
This support will be found in the upcoming Mesa 10.3.