Re: rx_copybreak value for non-i386 architectures

[Alan Cox]

> for non-i386 architectures.  Once I thought I understood it and it seems
> related to cache line alignment.  However, I am not sure exactly about the
> reason now.  Can someone enlighten me a little bit?

A lot of pci net cards can only start packets on a 4 byte boundary. A lot
of CPU's need 4 byte aligned read/writes for performance. The ethernet header
is howerver 14 bytes long.

For CPU's with poor unaligned performance it turns out better to copy or
copy/checksum the data so the IP/TCP headers are aligned. If the card can
hit 16bit boundaries then you will see card drivers doing

	alloc_skb(blah)

	skb_reserve(skb, 2);

to align the header of he buffer so that the IP data is aligned