From: Govindarajulu Varadarajan

dma_map_single()/dma_unmap_single() is more expensive than alloc_skb & memcpy
for smaller packets. By doing this we can reuse the dma buff which is already
mapped. This is very useful when iommu is on. The default skb copybreak value
is 256.

When iommu is on, we can go much higher than 256. All the drivers that supports
rx_copybreak provides module parameter to change this value. Since module
parameter is the least preferred way for changing driver values, this series
adds ethtool support for setting rx_copybreak.

Is there any mileage in having a system-wide default for rx_copybreak?
I'd have thought that the value is really driver independent since it
(mostly) depends on the comparative cost of dma_map and copy operations.

The same also applies to any equivalent copy done during transmit.
(Not sure if I've seen any drivers that keep a permanently mapped
buffer for transmitting small fragments.)

	David