82598EB Datasheet, PDF (326/596 Pages) Intel Corporation – Intel® 82598EB 10 Gigabit Ethernet Controller Datasheet

Intel^® 82598EB 10 GbE Controller - Register Descriptions
4.4.3.5.6 Receive Descriptor Control – RXDCTL (0x01028 + 0x40*n[n=0..63];
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
0x01
Write-Back Threshold
24:23
0x00
Reserved
25
0b
Receive Queue Enable
When set, the Enable bit enables the operation of the specific receive queue, upon
read – get the actual status of the queue (internal indication that the queue is
actually enabled/disabled).
26
0b
Received
31:27
0x00
Reserved
The register controls the fetching and write-back of receive 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, receive descriptors in the on-chip descriptor buffer. If the number drops
below PTHRESH, the algorithm considers pre-fetching descriptors from host memory. The host memory
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 receive descriptors. This threshold refers to the number
of receive descriptors in the on-chip buffer which 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.
Possible values:
PTHRESH = 0..32
WTHRESH = 0..16
HTHRESH = 0, 4, 8
Note: For proper operation, the PTHRESH value should be larger than the number of buffers needed
to accommodate a single packet/TSO.
Note:
Since the default value for write-back threshold is one, descriptors are normally written back
as soon as one descriptor is available. WTHRESH must contain a non-zero value to take
advantage of write-back bursting capabilities.
326