It looks like Imagination is trying to land their user space drivers for Rogue GPUs in mesa3d: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15...

In parallel, they're working on upstreaming the kernel bits too: https://gitlab.freedesktop.org/frankbinns/powervr/-/tree/pow...

Their ISA is here: http://cdn.imgtec.com/sdk-documentation/PowerVR+Instruction+...