On the xorg mailing list Benjamin Herrenschmidt posted "Radeon mem map fix(#2): Need regression tests please", that includes a patch that hopefully makes the "radeon memory map finally sane, removing all sort of hacks, and possibly fixing all sort of weird bugs".

Well, here's a modularized version of this patch, and here's a little overlay for bleeding-edge gentoo users that includes this patch. Use it at your own risk, you might end up having to restart your system, or worse... or get something fixed. Please follow the mailing list for details, I'm just publicizing an ebuild and modularized patch of this.