question from ignorance - Why not use Metal? Is that too embedded in the macOS system to be useful for something like Linux? Or is this for the sake of understanding the bare metal?
If you used metal as the graphics api on Linux, literally no existing Linux software would work with it unless you also used a layer like MoltenGL or MoltenVK (which have been written for a Mac system and would likely need modification). Linux graphics drivers also tend to have extra APIs for buffer management for X11/Wayland, which a molten compat layer probably doesn't do as molten is meant to run in-process with each app I believe.
Some of the Metal APIs are also a little intertwined with swift and objc.
There are no Metal implementations to use outside of Apple's proprietary software. But even if you had one, it would be useless on Linux for the existing software stack.
