GTT explanation request

I was told to post it here to remind Michel about it ^^"

Hi Michel...

I'm tinkering around in radeon DRM code as a hobby so if you have a

couple of minutes, could you explain, how GTT works for Radeon module? I've seen mentions of it here and there on the Internet, and tried to ping developers on IRC, but the overall picture is still unreachable for me.

I also looked through the code in radeon_object.c, radeon_ttm.c and

radeon_gem.c. In my mind, the GEM and the TTM do the same thing, so why is there a need for GEM and TTM coupled?..

Sorry if it's too much for asking