On Fri, May 25, 2012 at 7:56 AM, Bjorn Helgaas <bhelgaas@xxxxxxxxxx> wrote:
> On Thu, May 24, 2012 at 3:24 AM, Zhi Yong Wu <zwu.kernel@xxxxxxxxx> wrote:
>> On Thu, May 24, 2012 at 5:21 PM, Stefan Hajnoczi <stefanha@xxxxxxxxx> wrote:
>>> On Thu, May 24, 2012 at 9:56 AM, Zhi Yong Wu <zwu.kernel@xxxxxxxxx> wrote:
>>>> On Thu, May 24, 2012 at 4:37 PM, Stefan Hajnoczi <stefanha@xxxxxxxxx> wrote:
>>>>> On Thu, May 24, 2012 at 9:26 AM, Zhi Yong Wu <zwu.kernel@xxxxxxxxx> wrote:
>
>>>>>> By the way, someone usually mentioned pci host bridge, pci root
>>>>>> bridge. What the differences among them are?
>>>>>
>>>>> I'm not sure what the exact difference is. PCI "root" bridge refers
>>>>> to the hierarchical nature of PCI, while PCI "host" bridge refers to
>>>> You mean that PCI "root" bridge is one alias of another some bridge?
>>>
>>> The root bridge is the root of a tree of PCI busses. It's the bus
>>> that the host interacts with - there may be PCI bridges that attach
>>> additional busses.
>
> "Root bridge" and "host bridge" mean the same thing in Linux: a bridge
> that connects host CPUs to a PCI fabric. This bridge is not
> architected. The upstream interface and the programming interface are
> implementation specific. The downstream interface is PCI, so that
> part is standard, but since the programming interface is not defined,
> you can't *do* anything useful to it without an
> implementation-specific driver.
>
> On x86 and ia64, that driver is an ACPI driver, and the BIOS supplies
> ACPI methods that hide the implementation details from the OS.
>
> The PCI and ACPI specs use the term "host bridge," so that's what I
> prefer. The ACPI driver is "pci_root.c" and it calls it "PCI Root
> Bridge."
>
> A PCI bus immediately below a host bridge is commonly called a "root
> bus." This may be bus 0, but it need not be. Most modern systems
> have multiple host bridges, each of which has a root bus.
>
> A PCI-to-PCI bridge is different: it has PCI interfaces on both
> upstream ("primary") and downstream ("secondary") sides. It is
> defined by the spec.
>
>>> BTW I don't know the terminology, maybe it's PCI "domains" not
>>> "busses". Again, you'd have to look at the PCI specification to get
>>> the exact definition.
>
> Linux uses "domain" to refer to what the PCI and ACPI specs call a
> "segment group." Here's the definition (from the PCI Firmware Spec):
>
> "A number in the range 0…65535 that uniquely selects a PCI Segment Group.
>
> PCI Segment Group is purely a software concept managed by system
> firmware and used by the operation
> system. It is a logical collection of PCI buses (or bus segments).
> There is no tie to any physical entities. It is a
> way to logically group the PCI bus segments and PCI Express Hierarchies.
>
> PCI Segment Group concept enables support for more than 256 buses in a
> system by allowing the reuse of the PCI bus numbers. Within each PCI
> Segment Group, the bus numbers for the PCI buses must be unique. PCI
> buses in different PCI Segment Group are permitted to have the same
> bus number.
>
> A PCI Segment Group contains one or more PCI host bridges."
Great, very very clear, it is very helpful to me, maybe plus other
guys, thanks a lot.
>
> Bjorn
--
Regards,
Zhi Yong Wu
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