There is PowerVR SGX source code from around a 2011 state that's been leaked to the public. While some are happy for the source getting out there, it won't do much good as Luc confirms -- it will just taint any developers from being able to write a proper, open-source PowerVR graphics driver. Luc's post also criticizes the PowerVR hardware with how heavily microcode driven it is and really isn't worth reverse-engineering either.
Luc also dismisses Imagination's recent PowerVR Rogue ISA release.
While the state of Linux driver support for PowerVR hardware remains poor, Luc encourages Imagination Technologies to open-source their driver stack and documentation. His blog post ends with, "Given the structure of the IMG graphics driver stack, system integrators have a limited level of satisfaction with IMG. I really doubt that this has improved too much since my Nokia days. Going open source now, by actively supporting some clued open source developers and by providing extensive NDA-free documentation, should not pose much of a legal or political challenge anymore, and could massively improve the perception of Imagination Technologies, and their hardware. So go for it, IMG. No-one else is going to do this for you, and you can only gain from it!"