https://bugs.freedesktop.org/show_bug.cgi?id=35998
--- Comment #20 from Marek Olšák maraeo@gmail.com 2011-05-28 13:13:33 PDT --- The ColorTiling option only affects the buffers allocated by the DDX, which is usually just the window framebuffer. The textures and private framebuffers allocated by r300g are usually tiled regardless of the ColorTiling option.
Setting this environment variable disables tiling in r300g:
RADEON_DEBUG=notiling
It must be set such that the Gnome Shell can see it, of course.