Revision: 2.85

Specification Update

September 2012

34

1.5.5

Software Clarifications

Applies to Intel drivers.

1. While In TCP Segmentation Offload, Each Buffer is Limited to 64 KB

Clarification: The 82576 supports 256 KB TCP packets; however, each buffer is limited to 64 KB since

the data length field in the transmit descriptor is only 16 bits. This restriction increases

driver implementation complexity if the operating system passes down a scatter/gather

element greater than 64KB in length. This can be avoided by limiting the offload size to

64 KB.

Investigation has concluded that the increase in data transfer size does not provide any

noticeable improvements in LAN performance. As a result, Intel network software drivers

limit the data transfer size in all drivers to 64 KB.

Please note that Linux operating systems only support 64 KB data transfers.

For further details about how Intel network software drivers address this issue, refer to

Technical Advisory TA-191.

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2.

Serial Interfaces Programmed By Bit Banging

to perform consecutive register writes with a minimum delay between them. However,

simply inserting a software delay between the writes can be unreliable due to hardware

delays on the CPU and PCIe interfaces. The delay at the final hardware interface might be

less than intended if the first write is delayed by hardware more than the second write.

To prevent such problems, a register read should be inserted between the first register

write and the software delay, i.e. “write”, “read”, “software delay”, “write”.

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3.

PF/VF Drivers Should Configure Registers That Are Not Reset By VFLR

Clarification: The following registers are not reset by VFLR and need to be configured by PF or VF in

case of a change to a new configuration (such as VF OS transition):

RDH/T, TDH/T, PSRTYPE, SRRCTL, RXDCTL, TXDCTL, TDWBAL/H, RXCTRL, TXCTRL

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