From: Mike Frysinger <redacted>
Date: Fri, 29 May 2009 07:20:18 -0400

that isnt how the DMA is being used here.  there is no register to
read back because the DMA controller is doing the reading indirectly.
but along those lines, it's not entirely clear which DMA registers
this SSYNC is supposed to be protecting because all of the updates are
happening on the descriptors in external memory, not the DMA registers
themselves.  i'll have Sonic clarify.

Yes, that's really confusing because the commit message says
"registers".