Device-to-device memory-transfer offload with P2PDMA
Device-to-device memory-transfer offload with P2PDMA
One of the most common tasks carried out by device drivers is setting up DMA operations for data transfers between main memory and the device. Often, data read into memory from one device will be immediately written, unchanged, to another device. Common examples include carrying the image between the camera and screen on a mobile phone, or downloading files to be saved on a disk. Those transfers have an impact on the CPU even if it does not use the data directly, due to higher memory use and effects like cache trashing. There are cases where it is possible to avoid usage of the system memory completely, though. A patch set (posted by Logan Gunthorpe with contributions by Christoph Hellwig and Steve Wise) has been in the works for some time that addresses this case for PCI devices using peer-to-peer (P2P) transfers, with a focus on offering an offload option for the NVMe fabrics target subsystem.