On 14/03/18 12:51 PM, Bjorn Helgaas wrote: > You are focused on PCIe systems, and in those systems, most topologies > do have an upstream switch, which means two upstream bridges. I'm > trying to remove that assumption because I don't think there's a > requirement for it in the spec. Enforcing this assumption complicates > the code and makes it harder to understand because the reader says > "huh, I know peer-to-peer DMA should work inside any PCI hierarchy*, > so why do we need these two bridges?" Yes, as I've said, we focused on being behind a single PCIe Switch because it's easier and vaguely safer (we *know* switches will work but other types of topology we have to assume will work based on the spec). Also, I have my doubts that anyone will ever have a use for this with non-PCIe devices. A switch shows up as two or more virtual bridges (per the PCIe v4 Spec 1.3.3) which explains the existing get_upstream_bridge_port() function. In any case, we'll look at generalizing this by looking for a common upstream port in the next revision of the patch set. Logan
