Hard Processor System Technical Reference Manual: Agilex™ 5 SoCs

ID 814346
Date 11/27/2024
Public
Document Table of Contents

5.1.6.2.1. Application Bus Burst Access

When configured through bit 0 of the DMA_SysBus_mode register, the DMA attempts to execute the fixed-length burst transfers on the host interface. The PBL field of transmit control and receive control registers of respective DMA channel indicates and limits the maximum burst length. The RX and TX descriptors are always accessed in the maximum possible (limited by packet burst length (PBL) or 16 * 8/bus width) burst length for 16 bytes to be read.

The TX DMA initiates a data transfer only when sufficient space is available in the MTL TX queue to accommodate the minimum of the following:

  • Bytes corresponding to the configured burst (PBL * bus_width/8)
  • Remaining bytes in TX Buffer without end of packet (EOP)
  • Number of bytes till EOP

The DMA indicates the start address and the number of transfers required to the host interface. When the host interface is configured for fixed-length burst, it transfers the data using the best combination of INCR4, INCR8, or INCR16 or higher if enabled and SINGLE transactions. Otherwise (no fixed-length burst), it transfers the data using any INCRx transactions.

The RX DMA initiates a data transfer in the following conditions:

  • Sufficient data is available in MTL RX queue to accommodate the configured burst.
  • EOP (when it is less than the configured burst length) is detected in the RX queue (possible only in store and forward mode).

The DMA indicates the start address and the number of transfers required to the host interface. If EOP is reached before the fixed-burst ends on the host interface, dummy transfers are performed in order to complete the fixed-burst. Otherwise (bit 0 of DMA_SysBus_mode register is reset), the DMA transfers the data using INCRx transactions.

When the host interface is configured for address-aligned beats, both DMA engines ensure that the first burst transfer initiated by the host interface is less than or equal to the size of the configured PBL. Therefore, all subsequent beats start at an address that is aligned to the configured PBL. The DMA can only align the address for beats up to 256 size because the host interface does not support more than BLEN 256.