Arria V Hard Processor System Technical Reference Manual

ID 683011
Date 12/02/2022
Public
Document Table of Contents

18.4.1. DMA Master Interface

The DMA interface acts as a bus master on the system interconnect. Two types of data are transferred on the interface: data descriptors and actual data packets. The interface is very efficient in transferring full duplex Ethernet packet traffic. Read and write data transfers from different DMA channels can be performed simultaneously on this port, except for transmit descriptor reads and write-backs, which cannot happen simultaneously.

DMA transfers are split into a software configurable number of burst transactions on the interface. The AXI_Bus_Mode register in the dmagrp group is used to configure bursting behavior.

The interface assigns a unique ID for each DMA channel and also for each read DMA or write DMA request in a channel. Data transfers with distinct IDs can be reordered and interleaved.

The DMA interface can be configured to perform cacheable accesses. This configuration can be done in the System Manager when the DMA interface is inactive.

Write data transfers are generally performed as posted writes with OK responses returned as soon as the system interconnect has accepted the last beat of a data burst. Descriptors (status or timestamp), however, are always transferred as non-posted writes in order to prevent race conditions with the transfer complete interrupt logic.

The slave may issue an error response. When that happens, the EMAC disables the DMA channel that generated the original request and asserts an interrupt signal. The host must reset the EMAC with a hard or soft reset to restart the DMA to recover from this condition.

The EMAC supports up to 16 outstanding transactions on the interface. Buffering outstanding transactions smooths out back pressure behavior improving throughput when resource contention bottlenecks arise under high system load conditions.