second look at topology commit domain logic

[guruofquality]

From @guruofquality on September 18, 2014 17:33

The man in the middle case (for example) opencl->opencl (same domain) with a reader block in the middle should probably still allow for a custom buffer manager that causes a DMA -- rather than causing a copy.

Copied from original issue: pothosware/pothos-library#73