VSC7511 datasheet(142/324 Pages) MICROSEMI

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Functional Descriptions

VMDS-10488 VSC7511 Datasheet Revision 4.2

127

•

The TOT_PAUSE flag is set.

•

The ingress port memory consumption exceeds the SYS:PORT:PAUSE_CFG.PAUSE_START

watermark.

The pause request is deasserted the following condition is met:

•

The ingress port’s consumption is below the SYS:PORT:PAUSE_CFG.PAUSE_STOP watermark.

3.11.8

Tail Dropping

The shared queue system implements a tail dropping mechanism where incoming frames are discarded

if the port’s memory consumption and the total memory consumption exceed certain watermarks. Tail

dropping implies that the frame is discarded unconditionally. All ports in the device are subject to tail

dropping. It is independent of whether the port is in flow control mode or in drop mode.

Tail dropping can be effective under special conditions. For example, tail dropping can prevent an ingress

port from consuming all the shared memory when pause frames are lost or when the link partner is not

responding to pause frames.

The shared queue system initiates tail dropping by discarding the incoming frame if the following two

conditions are met at any point while writing the frame data to the memory.

•

If the Ingress port memory consumption exceeds the SYS:PORT:ATOP_CFG.ATOP watermark

•

If the total consumed memory in the shared queue system exceeds the

SYS:PORT:ATOP_TOT_CFG.ATOP_TOT watermark

3.11.9

Test Utilities

This section describes some of test utilities that are built into the shared queue system.

Each egress port can enable a frame repeater (SYS::REPEATER), which means that the head-of-line

frames in the egress queues are transmitted but not dequeued after transmission. As a result, the

scheduler sees the same frames again and again while the repeater function is active.

The SYS:PORT:PORT_MODE.DEQUEUE_DIS disables both transmission and dequeuing from the

egress queues when set.

3.11.10 Energy Efficient Ethernet

This section provides information about the functions of Energy Efficient Ethernet in the shared queue

system. The following table lists the registers associated with Energy Efficient Ethernet.

The shared queue system supports Energy Efficient Ethernet (EEE) as defined by IEEE Draft P802.3az

by initiating the Low Power Idle (LPI) mode during periods of low link use. EEE is controlled per port by

an egress queue state machine that monitors the queue fillings and ensures correct wake-up and sleep

timing. The egress queue state machine is responsible for informing the PCS in the port module of

changes in EEE states (active, sleep, low power idle, and wake up).

Table 100 •

Energy Efficient Ethernet Control Registers

Description

Replication

DEV::EEE_CFG

Enables configuration of Energy Efficient Ethernet. Status

bit indicating that egress port is in the LPI state.

Per port

QSYS::EEE_CFG

Configures fast queues.

Per port

QSYS::EEE_THRES

Configures bytes and frame thresholds.

None

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