On Tue, Feb 04, 2014 at 07:34:50PM +0100, Arnd Bergmann wrote: > Well, the way I see it, we already have support for arbitrary > PCI domains in the kernel, and that works fine, so we can just > as well use it. That way we don't have to partition the available > 256 buses among the host bridges, and anything that needs a separate > PCI config space can live in its own world. Quite often when you > have multiple PCI hosts, they actually have different ways to > get at the config space and don't even share the same driver. > On x86, any kind of HT/PCI/PCIe/PCI-x bridge is stuffed into a > single domain so they can support OSs that only know the > traditional config space access methods, but I don't see > any real advantage to that for other architectures. Supporting a standard configration interface is a solid reason, but there is alot more going on. For instance to support peer-to-peer IO you need to have a consisent, non-overlapping set of bus/device/function/tag to uniquely route TLPs within the chip. Cross domain TLP routing in HW is non-trivial. IOMMUs (and SR-IOv) rely on the BDF to identify the originating device for each TLP. Multiple domains means a much more complex IOMMU environment. Failure to integrate on-chip devices into the PCI world also means thing like SR-IOv won't work sanely with on-chip devices. The only reason we should see multi-domain on a SOC is because the HW design was lazy. Being lazy misses the Big Picture where PCI is the cornerstone of many important Server/Enterprise technologies. > > SOC internal peripherals should all show up in the bus 0 config space > > of the only domain and SOC PCI-E physical ports should show up on bus > > 0 as PCI-PCI bridges. This is all covered in the PCI-E specs regarding > > the root complex. > > > > Generally I would expect the internal peripherals to still be > > internally connected with AXI, but also connected through the ECAM > > space for configuration, control, power management and address > > assignment. > > That would of course be very nice from a software perspective, > but I think that is much less likely for any practical > implementation. Well, all x86 implementations do this already.. It actually isn't that big a deal from a HW perspective, you just have to think about it fully, understand PCI, and position your registers accordingly. Jason -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
