why o why don't I get an interrupt when SGDMA engine completes a descriptor or chain

Are you certain that you have created the linked-list descriptor chain correctly since if there is a problem there then the SGDMA might go into the weeds and never complete. Also in case you didn't notice, the SGDMA slave port places the status register at byte offset 0 and the control register at byte offset 16. So perhaps you were communicating with the control register assuming it was at offset 4 (i.e. maybe the interrupt mask wasn't enabled).