Raspberry Pi VC4
Using the "VC4" driver on the raspberry pi to enable hardware acceleration (in X, Wayland, opengl applications) presents many challenges. There's plenty of instructions floating around for Raspbian, but for Gentoo, not so much. This page contains a couple of nuggets of wisdom that may help you get there.
To have proper GPU acceleration with VideoCore, you need its kernel module loaded. That module has been integrated in v4.5, but it's also present in rpi's kernel v4.4.
Mesa and friends
Before enabling kernel module and switching RPi to GPU you need to rebuild media-libs/mesa with support of VideoCore4. To do so add following to your /etc/portage/make.conf:
And rebuild media-libs/mesa.
It is also a good idea to add
egl gles1 gles2 as global use flags.
Enabling the VC4 kernel module
Once you have mesa VC4 support you can enable kernel module and try to switch to VC4 GPU.
To enable the kernel module, you have to enable the optional overlay in your
dtoverlay=vc4-kms-v3d. It's also recommended to bump your
gpu_mem to 128 (it's what
raspi-config does when you enable the driver through it).
Make sure that you've installed latest version of sys-boot/raspberrypi-firmware or you'll have a blank screen!
Also, you should know that because that driver is a kernel module, you won't have any HDMI output until it's loaded, which is somewhere in the middle of your init procedure.
Is my module loaded?
Once you're booted up, you can verify that your module is properly loaded with
lsmod. You're supposed to have a
vc4 module in there. Also, it's important to verify that
/dev/dri/card0 exists. If you don't have that device, nothing's going to work.
Is it working?
To test your setup, you need to install
x11-apps/mesa-progs. This provides
glxgears, which you can run with
glxgears -info. If you have something like
GL_RENDERER = Gallium 0.4 on VC4 in the output, then it's a success! If it's using
llvmpipe, you're almost there. If it's using the software rasterizer, you're not there.