The MacBookPro15,1 finally has hybrid graphics

The MacBookPro15,1 normally boots Linux with its AMD GPU in charge of the display. It works, it is fast and it uses around 24 watts while doing almost nothing. Roughly half of that disappears when the discrete GPU is actually allowed to turn off.

That missing half is why hybrid graphics mattered so much.

With Intel as the primary GPU, the desktop runs on the integrated graphics and the AMD GPU can stay in D3cold. An application using DRI_PRIME=1 wakes it on demand. An external monitor wakes it because the display outputs are connected there. Close the application or unplug the monitor and it goes back to sleep.

That is how everyone expects a dual-GPU laptop to behave. It just took quite a lot of work to make this particular one agree.

Turning it off was the easy part#

Apple GMUX can cut power to the GPU. The hard part was getting the Polaris GPU back after that happened. The normal GMUX sequence left its PCI configuration space inaccessible. Following the additional link transitions exposed by the firmware made power-on reliable on both the 2018 and 2019 revisions.

Before runtime switching was possible at all, AMDGPU also needed a reset quirk for this machine. Without it the GPU's SMU could remain dead after resume. That smaller foundation started with an idea and rough patch from @FrederickGeek8. I turned it into an upstream submission, and roughly one day later it was merged upstream before the larger hybrid series was ready.

AMDGPU still had to learn that GMUX is a valid runtime power method. Linux already understood other laptop power schemes, just not this one.

The missing GMUX power-on sequence was developed with @byte. It became the platform side of the final series and the bit that made the Polaris GPU return reliably instead of disappearing behind inaccessible PCI configuration space.

Then system suspend woke the powered-off GPU again for its HDMI audio function. That was the final surprise: the graphics fix needed a small ALSA change so an audio device attached to a GPU that is physically off can remain off during suspend.

Three subsystems, one feature.

Making it convenient to use#

The firmware preference for the boot GPU lives in an Apple NVRAM variable. Expecting users to discover and write that variable by hand would turn a working kernel feature into trivia for people who already know the answer.

KAIT2EN therefore installs T2 Hybrid GPU Control on the 15,1. It selects the integrated boot GPU, shows whether the discrete GPU is in DynOff or DynPwr and leaves ordinary PRIME offload to the desktop and applications.

The feature has been tested with repeated wakeups, suspend and resume, and a collection of Thunderbolt and USB-C displays. It is now our default path for this model.

Upstream is where it should live#

The downstream version requires KAIT2EN to rebuild parts of AMDGPU whenever Fedora updates the kernel. That is inconvenient for users and a maintenance job we do not want forever.

The three-patch series has now been sent upstream to the platform, AMDGPU and ALSA maintainers. Review already improved the HDA integration and caught a PCI device lifecycle problem in the GMUX patch. It also produced several revisions in one afternoon, including one correction after we checked what Apple's ACPI methods really return instead of assuming.

That process can look messy from outside. It is still better than carrying the same private kernel patch for years. The goal is a future Fedora kernel where the 15,1 simply has hybrid graphics, whether KAIT2EN is installed or not.

The current series is available in the kernel mailing list archive.

Phoronics picked it up the same day#

Actually impressive how fast these guys are. And also the fact that they picked it up shows that hybrid graphics support on Apple MacBooks is a desperately wanted feature by users. Also I believe this will be an interesting find for all other muxed Macbooks like T1 etc.., Here is the link to the Phoronics article