you are right, I've another question regarding the function
dma_wait_for_async_tx from async_tx.c, here is the body of the code:
       /* poll through the dependency chain, return when tx is complete */
1.        do {
2.                 iter = tx;
3.                 while (iter->cookie == -EBUSY)
4.                         iter = iter->parent;
5.
6.                 status = dma_sync_wait(iter->chan, iter->cookie);
7.         } while (status == DMA_IN_PROGRESS || (iter != tx));

assume that:
- The interrupt capability is not provided.
- Request A was sent to chan 0
- Request B that depends on A is sent to chan 1
- Request C that depends on B is send to chan 2.
- Also, assume that when C is handled by async_tx_submit(), B is still
not queued to the dmaengine (cookie equals to -EBUSY).

In this case, dma_wait_for_async_tx will be called for C, now, it
looks for me that the do while will loop forever, even when A gets
completed. this is because the iter will point to B after line 4, thus
the iter != tx (C) will always become true.

saeed