Kernel environment variable hw.busdma.default can take values 'bounce'
and 'dmar' and selects corresponding busdma backend as default.
Per-device environment variable hw.busdma.pci<domain>.<bus>.<slot>.<func>
takes the same values and overrides hw.busdma.default for the given device.
Note that even with hw.busdma.default=bounce, DMA translation engines
are still started if DMARs are enabled, to disable them use
hw.dmar.dma tunable, as before.
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
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