BCM5722 Programmer’s Guide
10/15/07
Broadcom Corporation
Page 59 Receive Data Flow Document 5722-PG101-R
Section 6: Receive Data Flow
INTRODUCTION
The RX MAC pulls BDs from RX producer rings. The RX BD specifies the location(s) in host memory where packet data may
be moved. Figure 24 on page 60 shows the receive buffer descriptor cycle.
All ingress Ethernet frames are classified by the RX rules engine. A class ID is associated to each frame based on QOS
rules setup in the RX MAC (see “Receive Rules Setup and Frame Classification” on page 66). The Receive List Placement
and Receive List Initiator portions of the MAC architecture move BDs to the RX return rings; the class ID associated to the
packet is examined to route the BD to a specific RX return ring.
Once the packet is queued to the RX return ring, the device driver will wait for indication of the same through the status block
update and interrupt from the host coalescing engine. The host coalescing engine will update the status block and generate
a line interrupt or MSI (see “Host Coalescing” on page 177 for further details regarding interrupts) when a specified host
coalescence criteria is met. Once the interrupt is generated, the host device driver will service the interrupt. The ISR will
determine if new BDs have been completed on the RX Return Rings. Next, the device driver will indicate to the network
protocol that the completed RX packets are available. The network protocol will consume the packets and return physical
buffers to the network driver at a later point.
The BDs may then be reused for new RX frames. The device driver must return the BD to an RX producer ring. For this
purpose, the driver should fill out either the opaque field or index field of the Rx BD when inserting/initializing the BD in an
RX Producer ring. When the BD is returned by the device through Return Ring, the opaque or index data field of the BD will
be used by the driver to identify the BD in Producer Ring that corresponds to the Returned BD in Return Ring. The device
driver will then reinitialize the identified BD in Producer Ring with a new allocated buffer and replenish the Receive Producer
Ring with this BD.
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