Mobile GPU Switching With X.Org & Linux?
One of the most recent innovations on the mobile front has been integrating two graphics processors into a notebook but not for binding them together via SLI or CrossFire but for real-time GPU switching. This technology though isn't supported on Linux, at least not yet.
Intel's Centrino 2 and AMD's Puma platforms support having an integrated graphics processor and discrete graphics processor. The idea behind this is that when the notebook is running on battery power it can switch to using the IGP and turn the other GPU completely off in order to save power. While the performance of the Intel GMA X4500 HD isn't that bad, when these newest notebooks are connected to a power adapter the main graphics processor can switch to the discrete performance-oriented GPU. You get the best of both worlds by having a long battery life but still having the graphics capabilities to perform well with the Phoronix Test Suite or whatever 3D goodness catches your fancy.
This dynamic switching and powering down of unused GPUs isn't supported though by X.Org at this time. There were some comments made during XDS 2008 that Intel's X.Org team soon could get underway in supporting this new technology on Intel notebooks, but not much was said. Today though Red Hat's Adam Jackson has commented on the matter. Adam Jackson was the X.Org 7.4 / X Server 1.5 release manager and shares responsibility with David Airlie for the state of X in Fedora and Red Hat Enterprise Linux.
In this blog post, Adam talks about GPUs and the different ways through with these dual GPU notebooks are configured. Some of these notebooks have BIOS options for controlling the GPU to use while others expose both GPUs on the PCI bus the entire time. So far they haven't experienced any luck in benefiting from ACPI in this work. Aside from just recognizing the GPU that is currently communicating with the display, extensive work is also required within X.Org to make this real-time switching even feasible. Adam concludes with, "Getting this to work well should actually be a lot of fun, and there's lots of opportunity to sweep away old bad design and come up with something good."
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