82598EB Datasheet, PDF (253/596 Pages) Intel Corporation – Intel® 82598EB 10 Gigabit Ethernet Controller Datasheet

Intel^® 82598EB 10 GbE Controller - Transmit Functionality
Rsv (bit 3:2) – Reserved
DD (bit 0) – Descriptor Done
IDX (4)
This field holds the index into the hardware context table to indicate which of the eight per-queue
contexts should be used for this request.
POPTS (6)
RSV (bit 5) – Reserved
TXSM (bit 1) – Insert TCP/UDP Checksum
When 1b, TCP/UDP checksum is inserted. In this case TUCMD.LP4 indicates whether the checksum is
TCP or UDP. When DCMD.TSE is set TXSM must be set to 1b.
IXSM (bit 0) – Insert IP Checksum
This field indicates that IP checksum is inserted. In IPv6 mode, it must be reset to 0b.
If DCMD.TSE is set, and TUCMD.IPV4 is set, IXSM must be set to 1b.
PAYLEN (18)
This field indicates the total length in bytes of the large send packet.
PAYLEN is ignored if TSE is not set.
Note:
When a packet spreads over multiple descriptors, all the descriptor fields are only valid in the
1st descriptor of the packet, except for RS, which is always checked, and EOP, which is always
set at last descriptor of the series.
3.5.3.3.2 Transmit Descriptor Structure
The transmit descriptor ring structure is shown in Figure 3-26 each ring uses a contiguous memory
space. A pair of hardware registers maintains the transmit descriptor ring in the host memory. New
descriptors are added to the ring by software by writing descriptors into the circular buffer memory
region and moving the tail pointer associated with that ring. The tail pointer points 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.
Hardware maintains internal circular queues of 64 descriptors per queue to hold the descriptors that
were fetched from the software ring. The hardware writes back used descriptors just prior to advancing
the head pointer(s).
Descriptors passed to hardware should not be manipulated by software until the head pointer has
advanced past them.
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