I am talking about the layers below SDL. The layers that give rise to questions like will the graphics card be supported on my favorite OS if I buy this new computer?. These layers are responsible for detecting and configuring your adapters & monitors, setting the resolution, colour depth, interrupts for vsync, dma for fast frame copying, etc. If these things do not work right, you are in for a sad sad desktop experience, even if your CPU were fast enough to do all the 3D magic you want.

Hilariously, using software SDL for your framebuffer (at least on X.org) will be much slower than using OpenGL PBOs. Don't talk about hardware acceleration here -- all we're doing is transferring a finished frame to the display. At this point, we already need much more than the "small" amount of code to implement KMS. Can it break? Yes it can. Does it break? Yes it does. Graphics driver issues are still among the most common problems I witness people struggle with, as far as getting their OS to run smoothly goes.