82559 or later controllers added simple checksum calculation logic
in RU. For backward compatibility the computed checksum is appended
at the end of the data posted to Rx buffer. This type of simple
checksum calculation support had been used on several vendors such
as Sun HME/GEM, SysKonnect GENESIS and Marvell Yukon controllers.
Because this type of checksum offload support requires parsing of
received frame and pseudo checksum calculation with software
routine it still consumes more CPU cycles than that of full-fledged
checksum offload controller. But it's still better than software
checksum calculation.
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