82598EB Datasheet, PDF (222/596 Pages) Intel Corporation – Intel® 82598EB 10 Gigabit Ethernet Controller Datasheet

Intel^® 82598EB 10 GbE Controller - Receive Functionality
Table 3-53. Descriptor Read Format
63
0
Packet Buffer Address [63:1]
8
Header Buffer Address [63:1]
1
0
A0/
NSE
DD
Packet Buffer Address (64)
The physical address of the packet buffer. The lowest bit is either A0 (LSB of address) or NSE (No Snoop
Enable), depending on bit DCA_RXCTRL.RXdataWriteNSEn of the relevant queue.
Header Buffer Address (64)
The physical address of the header buffer. the lowest bit is DD (Descriptor done).
Note: The 82598 does not support null descriptors, in which a packet or header address is equal to
zero.
When software sets the NSE (No-snoop) bit, the 82598 places the received packet associated with this
descriptor in memory at the packet buffer address with the no-snoop bit set in the PCIe attribute fields.
NSE does not affect the data written to the header buffer address.
When a packet spans more than one descriptor, the header buffer address is not used for the second,
third, etc. descriptors; only the packet buffer address is used in this case.
No-snoop is enabled for packet buffers that the software device driver knows have not been touched by
the processor since the last time they were used, so the data cannot be in the processor cache and
snoop is always a miss. Avoiding these snoop misses improves system performance. No-snoop is
particularly useful when the data movement engine is moving the data from the packet buffer into
application buffers and the software device driver is using the information in the header buffer for
operation with the packet.
Note: When No-Snoop Enable is used, relaxed ordering should also be enabled with
CTRL_EXT.RO_DIS.
Note that when the 82598 writes back the descriptors, it uses the descriptor format shown in
Table 3-54. The SRRCTL[n].DESCTYPE must be set to a value other than 000b for the 82598 to write
back the special descriptors.
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