Prior to starting DMA operations, the software application must set up a descriptor chain. When the AXI DMA begins processing the descriptors, it fetches, processes, and then updates the descriptors. By analyzing the descriptors, the software application can read the status on the associated DMA transfer, fetch user information on receive (S2MM) channels, and determine completion of the transfer. With this information, the software application can manage the descriptors and data buffers.
Software applications process each buffer associated with completed descriptors and reallocate the descriptor for AXI DMA use. To prevent software and hardware from stepping on each other, a Tail Pointer Mode is created. The tail pointer is initialized by software to point to the end of the descriptor chain. This becomes the pause point for hardware. When hardware begins running, it fetches and processes each descriptor in the chain until it reaches the tail pointer. The AXI DMA then pauses descriptor processing. The software is allowed to process and re-allocate any descriptor whose Complete bit is set to 1.
The act of writing to the TAILDESC register causes the AXI DMA hardware, if it is paused at the tail pointer, to begin processing descriptors again. If the AXI DMA hardware is not paused at the TAILDESC pointer, writing to the TAILDESC register has no effect on the hardware. In this situation, the AXI DMA continues to process descriptors until reaching the new tail descriptor pointer location. Descriptor Management must be done by the software. AXI DMA does not manage the descriptors.