Is there a simple way for me to test whether the GART is working on a radeon driver?

I'm hacking the Radeon 200M, and I think that I've setup the GART properly. (unfortunately, the CP still doesn't work...)

Is there a way for me to test if the memory is mapped in properly?

Ideally, I would like some thing like the following:
1) Write Address & Value to some radeon register.
2) Read the value back from the DRM module.
3) See that the written value matches the read value.
4) Rejoice.