Raspberry Pi VC4
The Raspberry Pi 3 VC4 driver is not available on 64-bit (ARM64/AARCH64) systems. The Raspberry Pi foundation has stated "we are not working on this, and are unlikely to do so in the near future". Using the open source vc4-kms-v3d driver listed below instead is recommended
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.
Kernel
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.
To get VC4 core working you need to use latest firmware from sys-boot/raspberrypi-firmware 9999 ebuild. To have installed only firmware files and not kernel - follow this guide modifying ebuild file.
Mesa and friends
Before enabling kernel module and switching RPi to GPU you need to rebuild media-libs/mesa with support of VideoCore4 plus in case of a Raspberry Pi 4 VideoCore VI. To do so add following to your /etc/portage/make.conf:
/etc/portage/make.confVIDEO_CARDS="v3d vc4"
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 config.txt with 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.
If it is using llvmpipe and /usr/share/X11/xorg.conf.d/99-fbturbo.conf exists, try to remove Driver "fbturbo" from that file.