+
+Alignment vs. Networking
+========================
+
+On architectures that require aligned loads, networking requires that the IP
+header is aligned on a four-byte boundary to optimise the IP stack. For
+regular ethernet hardware, the constant NET_IP_ALIGN is used. On most
+architectures this constant has the value 2 because the normal ethernet
+header is 14 bytes long, so in order to get proper alignment one needs to
+DMA to an address which can be expressed as 4*n + 2. One notable exception
+here is powerpc which defines NET_IP_ALIGN to 0 because DMA to unaligned
+addresses can be very expensive and dwarf the cost of unaligned loads.
+
+For some ethernet hardware that cannot DMA to unaligned addresses like
+4*n+2 or non-ethernet hardware, this can be a problem, and it is then
+required to copy the incoming frame into an aligned buffer. Because this is
+unnecessary on architectures that can do unaligned accesses, the code can be
+made dependent on CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS like so:
+
+#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
+ skb = original skb
+#else
+ skb = copy skb
+#endif
+