Broadcom BCM5722 Programmer's Manual page 130

BCM5722
The host software must create an array of BD structures in host memory, referred to as a receive producer ring. Each receive
buffer descriptor within a producer ring describes, among other things, the location of a host memory buffer that is used to
store the packets received from the network. When the host software (as the producer) updates the mailbox register's
producer ring index that corresponds to the receive producer ring, the BCM5722 Ethernet controller automatically DMAs the
BD to itself from the host. When the DMA is completed, the BCM5722 Ethernet controller (as the consumer) updates the
status block's receive consumer ring index to signal it successfully consumed the BD. The BCM5722 Ethernet controller
keeps this BD in internal memory to know where to put a packet that is received from the network.
When a packet is received from the network, a BD gets updated with information regarding the received packet and the
packet is DMAed to a location in host memory described by the BD. The BCM5722 Ethernet controller (as the producer)
then updates the receive return ring producer index in the Status Block register corresponding to one of host memory's
receive return rings, and DMAs the BD to that receive return ring.
It is the responsibility of the host software to setup, initialize, and manage the data structures in host memory, namely the
receive producer rings and the receive return rings. The producer/consumer indices in the mailbox and status block are read
and updated by the host and BCM5722 Ethernet controller for this purpose.
R S S
ECEIVE IDE
O
VERVIEW
RSS is a scalable networking technology that enables receive packet processing to be balanced across multiple processors
in the system while maintaining in-order delivery of the data. The RSS enables packets from a single network adapter to be
processed in parallel on multiple CPUs/cores while preserving in-order delivery to TCP connections.
F D
UNCTIONAL ESCRIPTION
The figure below shows the processing of received packets when RSS is enabled. The RSS algorithm is based on a load-
sharing algorithm and performs the following steps.
• Computes a hash on the incoming packet to produce a 32-bit Hash result.
• Performs a lookup in the load balancing table (also called indirection table) using the one to seven least significant bits
of the Hash result to determine which of the n CPUs are processing the packet, where n is the number of CPUs
assigned to process received packets.
Adds a Base CPU Number to determine the exact CPU that will process the packet.
•
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