Data Sheet

Inline Functions—Ethernet Controller I210
297
When DCMD.TSE in TDESD is set, TXSM must be set to 1b.
If this bit is set, the packet should at least contain a TCP header.
IXSM, when set to 1b, indicates that IP checksum must be inserted. For IPv6 packets this bit must be
cleared.
If the DCMD.TSE bit is set in data descriptor, and TUCMD.IPV4 is set in context descriptor, POPTS.IXSM
must be set to 1b as well.
If this bit is set, the packet should at least contain an IP header.
7.2.2.3.9 PAYLEN (18)
PAYLEN indicates the size (in byte units) of the data buffer(s) in host memory for transmission. In a
single send packet, PAYLEN defines the entire packet size fetched from host memory. It does not
include the fields that hardware adds such as: optional VLAN tagging, Ethernet CRC or Ethernet
padding. When TCP or UDP segmentation offload is enabled (DCMD.TSE is set), PAYLEN defines the
TCP/UDP payload size fetched from host memory.
Note: When a packet spreads over multiple descriptors, all the descriptor fields are only valid in the
first descriptor of the packet, except for RS, which is always checked, DTALEN that reflects
the size of the buffer in the current descriptor and EOP, which is always set at last descriptor
of the series.
7.2.2.4 Transmit Descriptor Ring Structure
The transmit descriptor ring structure is shown in Figure 7-10. A set of hardware registers maintains
each transmit descriptor ring in the host memory. New descriptors are added to the queue by software
by writing descriptors into the circular buffer memory region and moving the tail pointer associated
with that queue. The tail pointer points to one entry beyond the last hardware owned descriptor.
Transmission continues up to the descriptor where head equals tail at which point the queue is empty.
Descriptors passed to hardware should not be manipulated by software until the head pointer has
advanced past them.