Having it off by default is a bad idea from a socket perspective. When you have 64 byte data packets consuming 1500+ bytes of data storage, which is what you get with copybreak disabled, TCP spends all of it's time copying packet data around as the socket buffering limits on receive are hit quite quickly due to the bad "real data to space used" ratio.
Does it really need to be particularly aggressive about that? How often are there great streams of small packets sitting in a socket buffer? One really only cares when the system starts getting memory challenged right? Until then does it really matter if there are 100 64 byte chunks of data sitting in 1500 byte buffers?
WRT drivers and avoiding expensive operations, indeed, there is a cut-off point where allocating a buffer and copying is cheaper than mapping a buffer for DMA. At least I've seen it on another platform and would suspect that while the syntax (cut-over point) will vary from platform to platform to OS etc the concept is indeed there.
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