[NateLawson]

There is no IOMMU in USB. You've got it backwards: the IOMMU in PC's is on the host side of the USB controller, not the device side.

There's an easy way to tell. Does the bus carry memory addresses? Then it supports DMA. Does it just send messages? No DMA to protect against.

The USB controller on a PC does support DMA. The OS device driver allocates buffers and passes them to the controller to fill. If it's properly programmed, it will only store data into those buffers. An IOMMU is there to prevent malicious kernel privileged code from bouncing through peripherals that support DMA to compromise other privileged code.

Messages on the USB bus side have no addresses in them and there is no DMA involved.

[pslam]

This is exactly what I'm saying.

"DMA" as the parent puts it implies a core in the same SoC, rather than external. It would be hair brained to let this have unfiltered access to the fabric. However, that's exactly how older SoCs used to do it - in fact it used to be in charge and the AP shoved behind the IOMMU.

These days nobody I know of is stupid enough to have that arrangement. So it's not really a choice of "DMA" vs USB. External isn't buying much, unless you distrust the fabric filter (IOMMU), which isn't necessarily paranoid... but a level beyond this kind of system decision.