[phendrenad2]

At a high level, it's actually really simple. Your PCIe devices are each given a region of the address space, say, 0x8428000000000000-0x8428000000000fff. Just write to that region from kernel mode. But what do you write? Well, that isn't standardized. It's not even really documented. The best documentation is the source code to the GPU drivers in the Linux kernel, which are usually added to by engineers working at GPU vendors, and they don't discuss it much.

[account42]

AMD does have some GPU register documentation for GCN at the bottom of https://developer.amd.com/resources/developer-guides-manuals... but not for RDNA / RDNA2.