Monday, January 11, 2021 [Tweets] [Favorites]

Dissecting the Apple M1 GPU

Alyssa Rosenzweig (via Hacker News):

Apple’s latest line of Macs includes their in-house “M1” system-on-chip, featuring a custom GPU. This poses a problem for those of us in the Asahi Linux project who wish to run Linux on our devices, as this custom Apple GPU has neither public documentation nor open source drivers.

[…]

The process for decoding the instruction set and command stream of the GPU parallels the same process I used for reverse-engineering Mali GPUs in the Panfrost project, originally pioneered by the Lima, Freedreno, and Nouveau free software driver projects. Typically, for Linux or Android driver reverse-engineering, a small wrap library will be written to inject into a test application via LD_PRELOAD that hooks key system calls like ioctl and mmap in order to analyze user-kernel interactions. Once the “submit command buffer” call is issued, the library can dump all (mapped) shared memory for offline analysis.

Previously:

Update (2021-01-22): Alyssa Rosenzweig (via Hacker News):

This week, I’ve reached a second milestone: drawing a triangle with my own open-source code. The vertex and fragment shaders are handwritten in machine code, and I interface with the hardware via the IOKit kernel driver in an identical fashion to the system’s Metal userspace driver.

[…]

These changes amount to around 1700 lines of code since the last blog post, available on GitHub. I’ve pieced together a simple demo animating a triangle with the GPU on-screen. The window system integration is effectively nonexistent at this point: XQuartz is required and detiling the (64x64 Morton-order interleaved) framebuffer occurs in software with naive scalar code. Nevertheless, the M1’s CPU is more than fast enough to cope.

Update (2021-07-02): See also: Part III (Hacker News) and Part IV.

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