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Functional Descriptions

VMDS-10492 VSC7440 Datasheet Revision 4.2

218

3.24.4.4 Priority-Based Flow Control

There is a fifth accounting scheme for the pending frames. It operates on ingress memory use, but has

the same set of thresholds as the other account systems. The output from that system is however not

used for stopping/discarding frame copies, but for requesting the port to transmit pause flow control

frames for the involved priorities. The threshold levels here should be configured in a way so that the

blocking thresholds cannot be reached during the trailing amount of data.

3.24.4.5 Threshold Configuration

There is a large number of thresholds for the reservations described. They are all found in the

QRES::RES_CFG register, which is replicated approximately 5,120 times.

The following table lists other registers involved in congestion control.

Ingress priority thresholds

Set to their maximum value, as the QoS classes should not have any sharing across

WRED groups.

Ingress color thresholds

Set to their maximum.

WRED group memberships

Set to define the WRED groups. For each group, define as well the up to 24 WRED

profiles (per QoS class and DP level)

Egress sharing thresholds

Set the QoS class thresholds to the highest possible consumption before any profile

hits 100% probability. The color thresholds can then operate on the remainder of the

resource in control and can all be set to the same value, allowing use of all the

remaining. A discard due to the WRED profile takes precedence over other sharing

states, so the area outside the group sections is only available for green traffic.

QoS reservation

(QRES::RES_QOS_MODE)

Disable for all.

Table 147 •

Threshold Configuration Overview

QRES::RES_CFG Replication Description

0 – 119

Ingress memory reservation for port P priority Q accessed at index 8P + Q.

496 – 503

Ingress memory shared priority threshold for priority Q accessed at index 496 + Q.

508 – 511

Ingress memory shared threshold for DP levels 1, 2, 3, and 0.

512 – 526

Ingress memory reservation for port P, shared between priorities, accessed at index

512 + P.

1,024 – 2,047

Ingress reference thresholds. Same organization as for ingress memory, but added

offset 1,024.

2,048 – 3,071

Egress memory thresholds. Same organization as for ingress memory, but added

offset 2,048.

3,072 – 4,095

Egress reference thresholds. Same organization as for ingress memory, but added

offset 3,072.

4,096 – 5,119

PFC memory thresholds, operating on ingress memory use. PFC is activated when

no memory left according to these thresholds. Same organization as the ingress

stop thresholds, but added offset 4,096.

Table 148 •

Congestion Control Registers Overview

Description

QRES::RES_STAT (replicated same

way as RES_CFG)

Returns maximum value the corresponding threshold has been compared

with.

Table 146 •

WRED Sharing Configuration (continued)

Configuration

Setting

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