Running kernel 3.9.x (3.9.4 now), git mesa, git ati driver I had no issues with
these adjustments:

# cat /etc/environment 
LIBGL_DRIVERS_PATH=/opt/xorg/lib/dri/
R600_DEBUG=nodma

and:

# git diff

diff --git a/src/gallium/winsys/radeon/drm/radeon_drm_winsys.c
b/src/gallium/winsys/radeon/drm/radeon_drm_winsys.c
index 15d5d31..5b1d0fb 100644
--- a/src/gallium/winsys/radeon/drm/radeon_drm_winsys.c
+++ b/src/gallium/winsys/radeon/drm/radeon_drm_winsys.c
@@ -399,6 +399,7 @@ static boolean do_winsys_init(struct radeon_drm_winsys *ws)
                                       &ws->info.r600_ib_vm_max_size))
                 ws->info.r600_virtual_address = FALSE;
         }
+        ws->info.r600_virtual_address = FALSE;
     }

     /* Get max pipes, this is only needed for compute shaders.  All evergreen+

(maybe not relevant)

No GPU hangs. No weird radeon related stuff in /var/log/messages.
I could even run bfgminer succesfully with tstellard's recent Cayman fixes for
llvm/OpenCL.

Bisecting got me nowhere. :-(

So what would be a next step?