[Bug 215003] apple_gmux fails to initialize and iGPU unclaimed on MacBook Pro 16" 2019

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=215003

--- Comment #5 from Lukas Wunner (lukas@wunner.de) ---
Hm, why are runtime services disabled? Are you using "noefi" or "efi=noruntime"
on the command line or is this perhaps an RT kernel? Could you attach full
dmesg output?

Looking at the ACPI dump I notice that GMUX only has an 8 byte Memory32Fixed
region:

    Name (_CRS, ResourceTemplate ()  // _CRS: Current Resource Settings
    {
        Memory32Fixed (ReadWrite,
            0xFE0B0200,         // Address Base
            0x00000008,         // Address Length
        )
    })

On my pre-retina MacBookPro9,1 it's a 256 byte Decode16 region:

    Name (_CRS, ResourceTemplate ()  // _CRS: Current Resource Settings
    {
        IO (Decode16,
            0x0700,             // Range Minimum
            0x07FF,             // Range Maximum
            0x01,               // Alignment
            0xFF,               // Length
            )
    })

Apple already changed the I/O interface once when they switched to retina
displays in 2012: Before, registers where accessed directly. After, registers
where accessed through a mailbox interface which required 3 32-bit registers
for communication. It looks like they changed the interface again, however now
there's only a memory region with 8 bytes, so only 2 32-bit registers. It will
be necessary to reverse-engineer the new interface (again) to get support for
GMUX working on these newer machines.

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