Linux 5.2 To Allow P2P DMA Between Any Devices On AMD Zen Systems
Written by Michael Larabel in AMD on 18 May 2019 at 07:14 AM EDT. 8 Comments
AMD --
With the Linux 5.2 kernel an AMD-supplied change by AMDGPU developer Christian K├Ânig allows for supporting peer-to-peer DMA between any devices on AMD Zen systems.

The patch by Christian allows for peer-to-peer DMA to happen on modern AMD systems even between different root ports, expanding the current assumptions by the kernel. The patch explains:
The PCI specs say that peer-to-peer DMA should be supported between any two devices that have a common upstream PCI-to-PCI bridge. But devices under different Root Ports don't share a common upstream bridge, and PCIe r4.0, sec 1.3.1, says routing peer-to-peer transactions between Root Ports in the same Root Complex is optional.

Many Root Complexes, including AMD ZEN, *do* support peer-to-peer DMA even between Root Ports. Add a whitelist and allow peer-to-peer DMA if both participants are attached to a Root Complex known to support it.

The change already landed in Linux Git as part of the PCI updates for Linux 5.2.
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