It's where the original changes were discussed
https://lkml.org/lkml/2011/5/24/227
It's where the specific questions were answered about tx_submit() and device_issue_pending()
http://www.gossamer-threads.com/lists/linux/kernel/1408064?search_string=tx_submit;#1408064
they were:
tx_submit is not supposed to start the transfer, it is supposed to add the descriptors in dma driver's queue.
The issue_pending is supposed to actually start the transfer.
The reasons were well articulated is discussion we had when we did the DMA documentation.
Pls see https://lkml.org/lkml/2011/5/24/530