Intel PCI User Manual

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When performing TCP segmentation, the packet prototype header initially transferred by DMA is 
stored internally and updated as each packet of the TCP segmentation operation is composed. As 

data for subsequent TCP segments is DMA’d into the Ethernet controller, the frame header for each 

segment is dynamically inserted in front of the frame payload data stream prior to being written to 
the packet buffer. In order to obtain the most efficient use of burst DMA operations, the transmit 

DMA attempts to fetch as much data from a descriptor as possible, rather than limiting itself to 

bursting each data segment individually. However, to do this, sufficient packet buffer space must 
be reserved to account for all headers are inserted into the fetched data stream, as the burst might 

span multiple data segments. The calculation of how much packet buffer space should be reserved 

is dependent on the MSS being used, the maximum-sized data buffer pointed to by a descriptor, 
and the current header size. This calculation is left to software to pre-calculate for the worst-case 

usage. The TSPBP register allows configuration of this buffer space that must be reserved as “pad” 

for worst-case header insertion.   To ensure that this value does not prevent descriptors from being 
serviced at all, it is necessary that the transmit packet buffer allocation should be larger than the 

sum of (maximum TCP HDRLEN + maximum MSS + TMPBP + 80 bytes).

Because the DMA attempts to issue burst fetches for a much data as possible, it is possible for the 

transmit DMA to cause the transmit packet buffer to approach fullness (less the pad specified). 
However, if the packet buffer empties slightly, the transmit DMA could initiate a series of small 

transfers. To further optimize the efficiency of the transmit DMA during TCP segmentation 

operation, the TSMT register allows configuration of the minimum number of bytes that the DMA 
should attempt to transfer in a single burst operation. The transmit DMA uses this value to refrain 

from issuing a burst read until at least TSMT bytes of data from the current data descriptor can be 

stored in the packet buffer. This check is ignored if, after a series of DMA operations, the 
descriptor contains a smaller number of unfetched data bytes. To ensure that this minimum 

threshold does not prevent descriptors from being serviced at all, it is necessary that the transmit 

packet buffer allocation should be larger than the sum of (TSMT + TSPBP + 80 bytes). To ensure 
compliance with PCI-X specifications, this value should not be programmed to exceed 4 KB (the 

largest single-burst transfer allowed by PCI-X).

The transmit DMA further refrains from initiating service of a new data descriptor unless sufficient 

packet buffer space exists to at least fetch a full data segment or complete a partially-fetched 
segment.