Descriptor Bypass Mode - 4.0 English

QDMA Subsystem for PCI Express Product Guide (PG302)

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4.0 English

Descriptor Bypass mode also supports crediting and status updates to user logic. In addition, Descriptor Bypass mode allows the user logic to customize processing of descriptors and status updates. Descriptors fetched by the descriptor engine are delivered to user logic through the descriptor bypass out interface. This allows user logic to pre-process or store the descriptors, if desired. On the descriptor bypass out interface, the descriptors can be a custom format (adhering to the descriptor size). To perform DMA operations, the user logic drives descriptors (must be QDMA format) into the descriptor bypass input interface.

If the user logic already has descriptors, which must be in QDMA format, it can be provided directly to the DMA through the descriptor bypass ports. The user logic does not need to fetch descriptors from the host if the descriptors are already in the user logic. The user logic should not send credits through the descriptor credit interface.