Andre Eikmeyer posted the patches today for this Apple GMUX hybrid graphics support in working to address a roughly 12 Watt power cost to booting the discrete GPU of the MacBookPro15,1 even when it's unused. Linux can currently boot with the integrated graphics but the limitation being addressed with these patches is the ability to bring the dGPU back-up after boot when needing to run more intensive graphics/gaming workloads.
Eikmeyer explained with the patches:
"The MacBookPro15,1 normally starts Linux with the discrete GPU as primary. On the machines tested here that results in roughly 24 W idle power draw, about 12 W of which is attributable to the otherwise unused discrete GPU. Selecting the integrated GPU at boot avoids that cost, but the discrete GPU cannot currently return after GMUX powers it down, so usable hybrid graphics and suspend are not available.
This series provides the missing power lifecycle across apple-gmux, amdgpu and the HDA controller. With the integrated GPU as primary, the discrete GPU then transitions between DynOff and DynPwr and offloading to the dGPU through DRI_PRIME is working as well as external monitor support.
The result has been tested for three weeks on both the 2018 and 2019 MacBookPro15,1 revisions. Runtime suspend and resume, system suspend and resume, repeated GPU wakeups and also a variety of external Thunderbolt and USB-C monitors were tested successfully. It is a huge step forward in terms of usability, convenience and battery life for everyone who tested."
The patches are now out for review on the Linux kernel mailing list.