bus_transaction_initiator - DMA configuration capability
Bus Transaction Initiators (BTIs) represent the bus mastering/DMA capability of a device, and can be used for granting a device access to memory.
Device drivers are provided one BTI for each bus transaction ID each of its devices can use. A bus transaction ID in this context is a hardware transaction identifier that may be used by an IOMMU (e.g. PCI addresses on Intel‘s IOMMU and StreamIDs on ARM’s SMMU).
A BTI can be used to pin and unpin memory used in a Virtual Memory Object (VMO). If a caller pins memory from a VMO, they are given device-physical addresses that can be used to issue memory transactions to the VMO (provided the transaction has the correct bus transaction ID). If transactions affecting these addresses are issued with a different transaction ID, the transaction may fail and the issuing device may need a reset in order to continue functioning.
TODO(teisenbe): Add details about failed transaction notification.