From: Robert Hancock <redacted>
Date: Fri, 26 Feb 2010 21:08:04 -0600

That seems like a reasonable approach to me. Only question is how to
implement the check for DMA_64BIT. Can we just check page_to_phys on
each of the pages in the skb to see if it's > 0xffffffff ? Are there
any architectures where it's more complicated than that?

On almost every platform it's "more complicated than that".

This is the whole issue.  What matters is the final DMA address and
since we have IOMMUs and the like, it is absolutely not tenable to
solve this by checking physical address attributes.