Quoth Jiri Slaby:
"It fixes mmap when IOMMU is used on x86 only, but breaks architectures
like ARM or PPC where virt_to_phys(dma_alloc_coherent) doesn't work.
We need there dma_mmap_coherent or similar (the trickery what
snd_pcm_default_mmap does but in some saner way). But this cannot be
done at this phase."