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 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.
A BTI manages a list of quarantined Pinned Memory Tokens (PMTs). If a PMT was
created from a BTI using
zx_bti_pin(), and the PMT's handle is released
zx_pmt_unpin() being called, the PMT will be quarantined.
Quarantined PMTs will prevent their underlying physical memory from being
released to the system for reuse, in order to prevent DMA to memory that has
since been reallocated. The quarantine may be cleared by invoking
TODO(teisenbe): Add details about failed transaction notification.