Building a Linux GPU Driver for the M4 Mac Mini in One Month

In a month-long endeavor, Niklas and the author successfully created a fully OpenGL ES 3.0 compliant GPU driver for the M4 Mac Mini and MacBook Neo. This process, which typically takes years, involved reverse engineering the AGX's intricate firmware ABI and user-space components using a transparent, verifiably clean room method. The driver is not yet ready for end users but the authors aim to release it as soon as possible.

The GPU is crucial for modern systems, as it enables efficient rendering. The goal of this project was to develop conformant OpenGL and soon Vulkan drivers for the specified Mac Mini and MacBook Neo models. Throughout the process, the authors refrained from using any Apple binaries, instead focusing on hardware traces and custom shaders.

User-space graphics reverse engineering was handled carefully, treating any required Apple blobs as opaque objects. The authors had a friend document these blobs to allow for a clean room implementation, which was mostly built by trial and error. The code, experiments, and documentation have been published for verification.

The project is divided into user and kernel space. The kernel driver interfaces with the firmware, allocates buffers, and manages scheduling, while user space understands how the GPU works and fills the buffers with data. In Apple Silicon, the kernel driver communicates with the GPU firmware, which runs a custom RTOS called RTKit.

The most challenging aspect of this project was reverse engineering the AGX's firmware ABI, which was far more complicated than the M1/M2's firmware ABI. The shared memory tree structure was highly complex, with firmware-owned fields intertwined with host-controlled fields. The authors drew inspiration from the work of Asahi Lina to understand the firmware ABI. They watched macOS's behavior, replayed it, and attempted to replicate it themselves, aided by the hypervisor.