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Transaction Attributes — Intel
®
82575EB Gigabit Ethernet Controller
324632-003 Intel
®
82575EB Gigabit Ethernet Controller
Revision: 2.1 Software Developer’s Manual and EEPROM Guide
January 2011 175
6.1.4.2 Relaxed Ordering
The 82575 takes advantage of the relaxed ordering rules of the PCIe* Specification.
Relaxed ordering can be used in conjunction with the no snoop attribute to allow the memory controller
to advance non-snoop writes ahead of earlier snooped writes.
Relaxed ordering is enabled in the 82575 by clearing the RO_DIS bit in the CTRL_EXT register. The
actual setting of relaxed ordering is done for LAN traffic by the host through the DCA registers.
Note: The 82575 cannot perform relax ordering for descriptor writes or an MSI write.
6.1.4.3 Snoop Not Required
The 82575 can be configured to set the Snoop Not Required attribute bit for master data writes.
System logic can provide a separate path into system memory for non-coherent traffic. The non-
coherent path to system memory provides a higher, more uniform bandwidth for write requests.
Note: The Snoop Not Required attribute does not alter transaction ordering. Therefore, to achieve
maximum benefit from snoop not required transactions, it is advisable to also set the
relaxed ordering attribute. This assumes that system logic supports both the snoop not
required and relaxed ordering attributes.
No snoop is enabled in the 82575 by clearing the NS_DIS bit in the CTRL_EXT register. The actual
setting of no snoop is done for LAN traffic by the host through the DCA registers.
6.1.4.3.1 No Snoop and Relaxed Ordering for LAN Traffic
Software configures non-snoop and relax order attributes for each queue and each type of transaction
by setting the respective bits in the DCA_RXCTRL and TCA_TXCTRL registers.
Table 52 lists the default behavior for the No Snoop and Relaxed Ordering bits for LAN traffic when I/
OAT 2 is enabled.
Table 52. LAN Traffic Attributes
Note: Rx payload no snoop is also conditioned by the NSE bit in the Receive descriptor.
6.1.4.3.2 No Snoop Option for Payload
Transaction No Snoop Default
Relaxed Ordering
Default
Comments
Rx Descriptor Read N Y
Rx Descriptor Write Back N N RO must never be
used for this traffic
Rx Data Write Y Y See note and section
below
Rx Replicated Header N Y
Tx Descriptor Read N Y
Tx Descriptor Write Back N Y
Tx Data Write N Y