Broadcom BCM5722 Programmer's Manual page 133

Programmer's Guide
10/15/07
Sec t io n 7 : Tran sm it Da ta F l ow
I
NTRODUCTION
Send Buffer Descriptors (BDs) begin on the Send Producer rings. The device driver updates the Mailbox to reflect available
Send BDs.
• The MAC moves the available Send BDs to device local memory—a cache.
• Next, the MAC selects a BD from the internal cache using priority scheduling.
The physical address, programmed in the Send BD by the host device driver prior to the Mailbox update, contains the host
memory location of the TX packet buffer. The MAC reads the address from Send BD and schedules a bus master DMA for
reading the packet data from host buffer. The packet data will be moved into device internal buffers from host buffers by
Read DMA engine, and all the read buffers of 1 packet are chained together into a cluster. This cluster is then sent to the
transmit MAC which sends the packet data to the integrated PHY for transmission on Ethernet media.
The write DMA engine will subsequently update the status block to indicate that the Send BD was consumed. The host driver
normally returns the packet buffers to the NOS/protocol so the next packet can reuse that host physical memory. The send
BD is now available for the next TX packet.
S R
END INGS
The send rings are shared data structures that are used to describe a series of data buffers that will be transferred onto the
network. The shared data structure is called the Ring Control Block (RCB), and the entries within a ring for describing the
data buffers are called the Send Buffer Descriptors (Send BDs).
Note: The maximum number of Send BDs (buffer descriptors) for a single packet is (0.75)*(ring size).
Associated with each ring are two indices that control its operation. These indices are the producer index and the consumer
index, which are not shared between the host software and the BCM5722 Ethernet controller. In the case of send rings, the
host software controls the producer index by adding elements (initializing a Send BD) to the ring. Similarly, the BCM5722
Ethernet controller controls the consumer index by removing elements (processing a Send BD) from the ring.
The host software is responsible for maintaining its producer index and updating it by writing to the send ring producer index
mailbox register. The mailbox registers are described in
offsets 0x200 through 0x3FF), and
actually triggers the BCM5722 Ethernet controller to process the send descriptors starting at its consumer index. As a
descriptor is processed, the consumer index is incremented, and the new index is reflected in a new status block update.
Status block is described in
When the producer and consumer indices are equal, the ring is empty. When the producer index is one behind the consumer,
the ring is full. Because of this configuration, the producer index always points to an empty slot. Thus, there will always be
at least one empty slot in a ring.
Document 5722-PG101-R
"Low-Priority Mailboxes" on page 324
"Status Block" on page 53.
Bro adco m Co rp or atio n
"Mailbox" on page 63, "High-Priority Mailboxes" on page 238
(for offsets 0x5800 through 0x59FF). The update
BCM5722
(for
Transmit Data Flow Page 74