This tutorial demonstrated transferring data from a p2p device to a non-p2p device (or p2p disabled device). The p2p-enabled device exposes its memory content to the PCIe and the non-p2p device exercises DMA-READ to transfer data directly from the PCIe Bus.
Fig 2: P2P Dataflow from p2p-enabled source device to destination device exercising DMA-Read of destination device
In case the data is needed to be transferred from a non-p2p device (or p2p disabled device) to p2p enabled device, the technique will almost remain the same. The p2p-enabled device still needed to expose its memory content to the PCIe bar, and the non-p2p device needed to exercise DMA-WRITE to send the data into the p2p device.
Fig 3: P2P Dataflow from source device to p2p-enabled destination device exercising DMA-Write of source device
A sample test case doing p2p transfer by exercising DMA-WRITE is provided inside the directory reference-files/design2.
DMA-Read vs DMA-Write: If both the devices have p2p enabled, then the user has a choice whether to use DMA-READ or DMA-WRITE to transfer the p2p buffer or in other words which device’s buffer to be declared as p2p. In that case transferring via DMA-WRITE can potentially show performance improvement over DMA-READ, hence you may choose the destination device’s buffer to be declared as p2p.