Stratix V Avalon-ST Interface for PCIe Solutions: User Guide

ID 683093
Date 5/03/2019
Document Table of Contents

11.1. Throughput of Posted Writes

The throughput of posted writes is limited primarily by the Flow Control Update loop as shown in Figure 1. If the write requester sources the data as quickly as possible, and the completer consumes the data as quickly as possible, then the Flow Control Update loop may be the biggest determining factor in write throughput, after the actual bandwidth of the link.

The figure below shows the main components of the Flow Control Update loop with two communicating PCI Express ports:

  • Write Requester
  • Write Completer

To allow the write requester to transmit packets continuously, the credit allocated and the credit limit counters must be initialized with sufficient credits to allow multiple TLPs to be transmitted while waiting for the FC Update DLLP that corresponds to the freeing of credits from the very first TLP transmitted.

You can use the RX Buffer space allocation - Desired performance for received requests to configure the RX buffer with enough space to meet the credit requirements of your system.