On Tue, Mar 13, 2018 at 05:21:20PM -0600, Logan Gunthorpe wrote: > On 13/03/18 05:08 PM, Bjorn Helgaas wrote: > > On Tue, Mar 13, 2018 at 10:31:55PM +0000, Stephen Bates wrote: > > If it *is* necessary because Root Ports and devices below them behave > > differently than in conventional PCI, I think you should include a > > reference to the relevant section of the spec and check directly for a > > Root Port. I would prefer that over trying to exclude Root Ports by > > looking for two upstream bridges. > > Well we've established that we don't want to allow root ports. I assume you want to exclude Root Ports because of multi-function devices and the "route to self" error. I was hoping for a reference to that so I could learn more about it. While I was looking for it, I found sec 6.12.1.2 (PCIe r4.0), "ACS Functions in SR-IOV Capable and Multi-Function Devices", which seems relevant. It talks about "peer-to-peer Requests (between Functions of the device)". Thay says to me that multi-function devices can DMA between themselves. > But we need to, at a minimum, do two pci_upstream_bridge() calls... > > Remember, we need to check that a number of devices are behind the same > switch. So we need to find a _common_ upstream port for several devices. I agree that peers need to have a common upstream bridge. I think you're saying peers need to have *two* common upstream bridges. If I understand correctly, requiring two common bridges is a way to ensure that peers directly below Root Ports don't try to DMA to each other. So I guess the first order of business is to nail down whether peers below a Root Port are prohibited from DMAing to each other. My assumption, based on 6.12.1.2 and the fact that I haven't yet found a prohibition, is that they can. > Excluding the multifunction device case (which I don't think is > applicable for reasons we've discussed before), this will *never* be the > first upstream port for a given device. If you're looking for a common upstream bridge, you don't need to make assumptions about whether the hierarchy is conventional PCI or PCIe or how many levels are in the hierarchy. You already have upstream_bridges_match(), which takes two pci_devs. I think it should walk up the PCI hierarchy from the first device, checking whether the bridge at each level is also a parent of the second device. Bjorn
