As mentioned in D20097 it's possible to address the issue of path-compressed radix trie collisions caused by multiple DMA mappings of the same physical address as well as make the common of case bounce-without-bounce almost as fast as it was before. This is accomplished by exposing whether or not the effective bus_dma implementation will simply be using an identity mapping or not.
N.b. this patch does not contain the required arm64 changes. I will implement those but I'm not sure I have the means to test them.