I just watched the new Video about the new AMD GPUs: https://www.youtube.com/watch?v=0xUioPsLRKA
Because I am currently developing an own bare metal driver for AMD Rx570 GPUs, I find minutes 7:18 to 8:00 of the video very interesting.
I also encountered the fact that the GPU has only a 256MB BAR of accessible VRAM at boot time. However, most GPUs have a PCI Express register for resizing the BAR to it's full size. The CPU can access all of the VRAM after resizing. This is done in Linux Kernel already since version 4 for every supported AMD GPU and not just the l