NVIDIA still isn't permitted to properly use DMA-BUF for buffer sharing between their binary driver and the open-source graphics drivers for being able to properly support the NVIDIA Optimus technology. GPL-only kernel symbols are blocking NVIDIA from tapping DMA-BUF and there's a few kernel developers who don't want these symbols to be used by NVIDIA's blob.
As the latest on this front concerning proper multi-GPU support under Linux and the drivers being able to cooperate with each other, NVIDIA's Aaron Plattner has put out a set of four patches for PRIME helpers. The patches courtesy of the veteran Linux developer at NVIDIA provide helper functions to abstract core parts of the GEM PRIME import and export functions for video memory management.
These helper functions allow concealing some low-level details from the GPU drivers while remaining optional for drivers to use. "A driver can use them by plugging in drm_gem_prime_import and drm_gem_prime_export into the drm_driver structure, or it can bypass them by plugging in its own functions. The first patch adds these helpers, and the later patches switch three drivers over to using them."
Aside from making the changes to the core DRM for supporting these PRIME helpers, Aaron went ahead and hooked up the Nouveau, Radeon, and Exynos drivers for using the helper functions, which leads to a net reduction in the lines of code added. There's just under 300 lines of new code while nearly 500 lines of code was chopped out by removing duplicated code.
As a description of the work from Aaron himself, "Instead of reimplementing all of the dma_buf functionality in every driver, create helpers drm_prime_import and drm_prime_export that implement them in terms of new, lower-level hook functions. These hooks are optional; drivers can opt in by using drm_gem_prime_import and drm_gem_prime_export as the .gem_prime_import and .gem_prime_export fields of struct drm_driver."
These PRIME helper patches are currently floating on dri-devel.