Section 5: Common Data Structures; Theory Of Operation; Descriptor Rings - Broadcom BCM5722 Programmer's Manual

Programmer's Guide
10/15/07
Se ction 5: Com mon Da ta Str uc t ur es
T O
HEORY OF PERATION
Several device data structures are common to the receive, transmit, and interrupt processing routines. These data structures
are hardware-related and are used by device drivers to read and update state information.
D R
ESCRIPTOR
In order to send and receive packets, the host and the controller use a series of shared buffer descriptor (BD) rings to
communicate information back and forth. Each ring is composed of an array of buffer descriptors that reside in host memory.
These buffer descriptors point to either send or receive packet data buffers. The largest amount of data that a single buffer
may contain is 65535 (64K-1) bytes (The length field in BD is 16 bits). Multiple descriptors can be used per packet in order
to achieve scatter-gather DMA capabilities.
Note: The maximum number of Send BDs for a single packet is (0.75)*(ring size).
There are three main types of descriptor rings:
• Send Rings
• Receive Producer Rings
• Receive Return Rings
Document 5722-PG101-R
INGS
Bro adco m Co rp or atio n
BCM5722
Common Data Structures Page 42