82598EB Datasheet, PDF (346/596 Pages) Intel Corporation – Intel® 82598EB 10 Gigabit Ethernet Controller Datasheet

Intel^® 82598EB 10 GbE Controller - Register Descriptions
TDT
Reserved
15:0
0x0
31:16
0x0
Transmit Descriptor Tail
Reads as 0x0. Should be written to 0x0 for future compatibility.
This register contains the tail pointer for the transmit descriptor ring. It points to a 16-byte datum.
Software writes the tail pointer to add more descriptors to the transmit ready queue. Hardware
attempts to transmit all packets referenced by descriptors between head and tail.
4.4.3.7.6 Transmit Descriptor Control – TXDCTL (0x06028 + n*0x40[n=0..31];
RW)
Field
PTHRESH
Reserved
HTHRESH
Reserved
WTHRESH
Reserved
Enable
Reserved
Reserved
Bit(s)
Initial
Value
Description
6:0
0x00
Pre-Fetch Threshold
7
0x00
Reserved
14:8
0x00
Host Threshold
15
0x00
Reserved
22:16
0x00
Write-Back Threshold
24:23
0x00
Reserved
25
0b
Transmit Queue Enable
When set, the Enable bit enables the operation of the specific transmit queue, upon
read – get the actual status of the queue (internal indication that the queue is
actually enabled/disable).
31:26
0b
Reserved
31:27
0x00
Reserved
This register controls the fetching and write-back of transmit descriptors. Three threshold values are
used to determine when descriptors are read from and written to host memory.
PTHRESH is used to control when a pre-fetch of descriptors is considered. This threshold refers to the
number of valid, unprocessed transmit descriptors the chip has in its on-chip buffer. If the number
drops below PTHRESH, the algorithm considers pre-fetching descriptors from host memory. This fetch
does not happen, however, unless there are at least HTHRESH valid descriptors in host memory to
fetch.
WTHRESH controls the write-back of processed transmit descriptors. This threshold refers to the
number of transmit descriptors in the on-chip buffer that are ready to be written back to host memory.
In the absence of external events (explicit flushes), the write-back occurs only after at least WTHRESH
descriptors are available for write-back.
Note: When WTHRESH = 0b, only descriptors with the RS bit set is written back.
346