Ingress refers to PCIe in the AXI direction. For PIO transfers from the host system to be accepted by the Endpoint, they have to hit the Endpoint BAR.
Since the host system is unaware of the AXI domain address on the Endpoint, an ingress translation is setup to map incoming BAR-hit transactions to AXI transactions. Before setting up the ingress translation aperture, the Endpoint software performs a handshake with the software driver running on the host system.
This example demonstrates a handshake using software interrupts and the scratchpad in DMA registers. A typical flow is illustrated in This Figure.