4 Debug Overview

Section 3 has a discussion on some initial debug checks to determine the status of basic send and receive of packets. A technique called follow-the-packet is introduced here which uses the Ethernet MAC interface statistics to follow the path that an Ethernet packet goes through for transmission and reception at the wire level. Figure 4-1 outlines the basic requirements to send and receive packets. The rest of this application note demonstrates how to analyze each stage of the block diagram.

Figure 4-1 Follow the Packet Flow

Fundamental steps of packet flow between two link partners:

1. Packets can flow only after an Ethernet link is established between the two link partners. As part of establishing the link, auto-negotiation is typically used to determine the best bit line rate and duplex mode. Remember, auto-negotiation is triggered upon the physical connection between Ethernet PHYs. In addition to link establishment, the Linux drivers must have initialized correctly for the Ethernet MAC, the MDIO interface and the respective Ethernet PHY. See also Network Driver Initialization Process.
2. After a link is established, the transmit path can be examined independently. This application note shows how to make sure that packets were sent on the wire and received by the link partner at the MAC level using MAC hardware statistics. It does not matter at this level what protocols are used, only that Ethernet packet were transmitted.
3. Like the transmit path, the receive path can be examined independently. This application note shows how packets are received at the MAC level by viewing the MAC hardware statistics at the DUT Ethernet interface receiving the packets from the link partner.

Figure 4-2 shows the connection between the Ethernet MAC and the Ethernet PHY. It is assumed here that the reader knows the function and purpose of the Ethernet MAC and the Ethernet PHY. The Ethernet MAC handles the sending and receiving of digital data and the PHY handles the converting of Ethernet Packets between the digital and analog domains.

Figure 4-2 Ethernet MAC and Ethernet PHY Interface

The components in Figure 4-2 show the data path (RX, TX), the control path (MDIO), and the error path or synchronization path between the Ethernet MAC and the Ethernet PHY. These are the most general signals needed between a MAC and PHY interface. Depending on the interface mode (MII, RGMII, RGMII, and so forth) between the MAC and PHY, there are different variations of these signals.