Mesa 21.2's Direct3D 12 Driver Now Allows For Easy Multi-GPU Selection
Mesa 21.2-devel has added support for the MESA_D3D12_DEFAULT_ADAPTER_NAME= environment variable so the D3D12 driver can select between multiple GPUs on a given system.
MESA_D3D12_DEFAULT_ADAPTER_NAME can be set with a substring of the given GPU you desire to use with the driver, whether it just be the GPU vendor name or a portion of the graphics card model, depending upon the GPU makeup of your system.
Without this environment variable or no matches, the first enumerated GPU is selected. The patch by one of Microsoft's engineers working on their Mesa/graphics efforts was merged this morning ahead of next quarter's Mesa 21.2 feature release.