The GPU is not the issue. There are open source OpenGL ES drivers available in upstream Mesa right now, that work with upstream kernel DRM driver, and for RPi4

The GPU is not the issue. There are open source OpenGL ES drivers available in upstream Mesa right now, that work with upstream kernel DRM driver, and for RPi4 users there's even a Vulkan driver. It's been pointed out elsewhere but briefly there is a VLIW processor (the "VPU") that is initially in charge of the entire boot sequence before handing off control to the main ARM cores; the bootcode.bin firmware for RPi devices is exactly this code. This includes things like bringing up PLLs and the on-board UART before handing off control to the ARM core where "userspace" code runs. There are many free RISC-V implementations, and several free GPU drivers for various hardware families, but there is no combination of the two in any meaningful sense right now. If I had to guess I'd say ImgTec is probably one of the ones you could expect to pop up in an SoC somewhere, since I doubt ARM or Qualcomm are going to license their GPUs outside their families... ImgTec recently started contributing some code to Mesa but otherwise have historically been pretty hostile. So the immediate speculation doesn't look great at the moment but who knows what could happen.