Descriptor RingsBCM5718 Programmer’s Guide
Broadcom®
January 29, 2016 • 5718-PG108-R Page 76
• The Length field specifies the length of the data buffer. The lengths for the buffers associated with a given
packet will add up to the length of the packet.
• The VLAN Tag field is only valid when the VLAN_TAG bit of Flags field is set. This VLAN Tag field contains
the 16-bit VLAN tag that is to be inserted into an IEEE 802.1Q (and IEEE 802.3ac)-compliant packet by the
controller. If VLAN tag insertion is desired, this field (and the flag) should be set in the first descriptor for
that packet (i.e., the descriptor that points to the buffer that contains the Ethernet header).
Large Segment Offload (LSO) Send BD
See “Large Segment Offload” on page 110.
8 CPU_PRE_DMA If set to 1, the controller’s internal CPU is required to act upon the packet before
the packet is given to the internal Send Data Initiator state machine. Normally
this bit should be set to 0.
9
CPU_POST_DMA
a
If set to 1, the controller’s internal CPU is required to act upon the packet before
the packet is given to the internal Send Data Completion state machine.
Normally this bit should be set to 0.
10 HDRLEN[3] The length of the Ether+IP+TCP Headers (combined) to be replicated in each
frame arising out of a Large TCP Segment (LSO). Maximum Header Length is
256B.
11 HDRLEN[4] –
12 HDRLEN[5] –
13 HDRLEN[6] –
14 HDRLEN[7] –
15
DON’T_GEN_CRC
a
If set to 1, the controller will not append an Ethernet CRC to the end of the frame.
a. Indicates that this bit should be set in all descriptors for a given packet if the desired capability is to be
enabled for that packet.
Note: The UDP checksum engine does not span IP fragmented frames.
Note: The Ethernet controller does not validate the value of the Length field and may generate an
error on the PCI bus if the Length field has a value of 0. The host driver must ensure that the Length
field is nonzero before enqueueing the BD onto the Send Ring.
Table 9: Defined Flags for Send Buffer Descriptors (Cont.)
Bits Name Description
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