Comment # 12 on bug 61182 from Alex Deucher

I think these are actually several bugs at play here.

1. CPU only has a 256 MB window into vram.  If the window is full due to
fragmentation or other mapped buffers, we fail.  Does disabling hyperz help? 
set env var R600_HYPERZ=0 (mesa 9.1 or older), or R600_DEBUG=nohyperz (mesa
git).

2. running out of GPU accessible memory when migrating buffers.  Does setting
radeon.gartsize=1024 on the kernel command line in grub help?