[PATCH 0/3] Apple GMUX hybrid graphics support for MacBookPro15,1

Hello everyone,

This series will enable hybrid graphics on MacBookPro15,1.

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.

So I have this one question regarding Apple's gpu-power-prefs EFI variable.
The firmware defaults to the discrete GPU, while selecting the integrated GPU
is required before this hybrid configuration can take effect. Would setting
that preference from the kernel be appropriate on a known-good model, or
should it remain a userspace policy decision? Some discoverable way to select
the power-efficient configuration would seem preferable to requiring users to
know about a vendor-specific NVRAM variable before Linux behaves like a hybrid
graphics laptop.

Thank you for your time and consideration.

Andre