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The kernel driver can write directly to registers and/or memory on the card. It also handles command packets generated by the userspace drivers and passes them to the hardware after checking the contents. Two types of packets are most commonly used -- type 0 packets are used to write registers from userspace (this is why the kernel driver checks 'em) and type 3 packets tell the chips to perform more complex commands.
Here's a subset of the commands supported on Evergreen -- dispatch commands are for compute, draw commands for graphics.