The general purpose DMA controller is located in the LPD with the RPU and moves data from a source to a destination on the interconnect. The controller includes an AXI4 interface to read and write memory with 16 word transfers for a total of 64 bytes per transaction.
The controller's two main programming modes are a simple register-based control and a flexible linked descriptor table mode. In descriptor mode, the controller autonomously fetches the descriptor tables from system memory.
The controller has eight separate channels that share common resources including the common buffer. Each channel can be independently enabled, paused, or disabled at any time. The pause functionality allows software to program a new sets of descriptors midway through the block memory transfer.
The DMA implements a 4 KB common data buffer that is shared by all eight channels. A controller structure is automatically managed by hardware where software enables and disables a channel without concern for the allocation of the common data buffer. Each channel uses the buffer on a first-come first-served basis. Buffer usage of each channel is controlled by programming registers for issue capability and rate control of a channel.
For descriptor based operation, the DMA controller implements independent source (SRC) and destination (DST) descriptors tables. The controller can transfer any size payload up to 1 GB with byte granularity. Descriptor payloads can start and end on any byte alignment.
For performance, the controller supports an optional over fetch feature. For memory sources that cannot support over fetch, the software can disable the feature on a per channel basis. Xilinx recommends only using this feature if it is supported by the source being read.