Jerome Glisse implemented support today in R600g for basic CS memory usage accounting, per this commit. The memory usage accounting basically comes down to making sure what's submitted to the GPU doesn't end up going over the video RAM and GTT size. Right now the check is seeing that the submission doesn't go over 70% the GTT+vRAM size to allow for some memory fragmentation.
The commit description explains, " The idea is to compute a gross estimate of memory requirement of each draw call. After each draw call, memory will be precisely accounted. So the uncertainty is only on the current draw call. In practice this gave very good estimate (+/- 10% of the target memory limit)."