[gentoo-user] anyone tried amdgpu (kernel module)

[Tsukasa Mcp_Reznor]

I have a Bonaire gpu, which has legacy support using the amdgpu kernel module.  I currently use the Radeon dri module with radeonsi mesa drivers and am quite happy.  But gentoo being gentoo I thought I'd give the amdgpu a go for the fun of it.
Tried a few variations and keep coming up with a black screen on boot and it's hard locked.  I'm using the same Firmware includes that the radeon driver requires, and from reading it looks like that's fine with amdgpu, so I'm not sure what else could be the issue.
Has anyone here tried and had success with it?  I've tried Kernels 4.2 4.2.4 4.3 and 4.3.2.  So I believe I'm missing something simple and it's not a kernel bug.