(pls wrap your email to 80lines, I have reflowed below for readability) 
> Hi Dan/Vinod,
> 
> I have one query regarding implementation of few routines in DMA drivers.
> 
> What the exact purpose of txd->tx_submit() and device_issue_pending() routines?
> 
> Should tx_submit() always start physical transfer (Obviously, if channel is
> free)? Or should it just queue requests and then finally a call to
> device_issue_pending() must start physical transfers?
(reflowed your email for 80lines)
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

~Vinod