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Just to add to Jordan's post for those that don't know. His picture shows an efficient way of reading from the DMA FIFO, first check to see if there are enough elements by requesting zero with a zero timeout, this will return the number elements remaining. Then only perform the actual read if the number of available elements is equal to (or more than) what you want. My understanding of this necessity is that although the transfer occurs via DMA, if there are not enough elements yet then the CPU actually polls (using a lot of resource) until the number of elements is achieved, and this can really kill performance.

You can read multiple elements from a DMA FIFO.