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    SubjectRe: [PATCH -v11 04/30] PCI: Add busn_res into struct pci_bus.
    On Mon, Apr 30, 2012 at 6:32 PM, Yinghai Lu <yinghai@kernel.org> wrote:
    > On Mon, Apr 30, 2012 at 4:25 PM, Bjorn Helgaas <bhelgaas@google.com> wrote:
    >>> --- a/include/linux/pci.h
    >>> +++ b/include/linux/pci.h
    >>> @@ -419,6 +419,7 @@ struct pci_bus {
    >>>        struct list_head slots;         /* list of slots on this bus */
    >>>        struct resource *resource[PCI_BRIDGE_RESOURCE_NUM];
    >>>        struct list_head resources;     /* address space routed to this bus */
    >>> +       struct resource busn_res;       /* track registered bus num range */
    >>>
    >>>        struct pci_ops  *ops;           /* configuration access functions */
    >>>        void            *sysdata;       /* hook for sys-specific extension */
    >>
    >> struct pci_bus already includes "secondary" and "subordinate".    This
    >> new "busn_res" looks like it will contain the same information.  Why
    >> do we need both?
    >
    > In some case the could be different.
    > for root bus from _CRS, busn_res could bigger than subordinate,
    > because scan_childbus will update subordinate.

    For a bus below a P2P bridge, I think it's always the case that the
    bridge's Subordinate Bus Number in config space == bus->subordinate ==
    bus->busn_res.end (correct me if I'm wrong). I don't like the
    redundancy in this case.

    For a root bus where you set bus->busn_res from _CRS and
    bus->subordinate = pci_scan_child_bus(), bus->busn_res.end will
    generally be different from bus->subordinate, but there's no point in
    keeping track of bus->subordinate.

    The reason we care about secondary and subordinate is so we can
    allocate bus numbers when enumerating devices behind a bridge. The
    only thing we need for that is the aperture of the upstream bridge and
    the apertures of any peer bridges on the same bus. Let's say we have
    this:

    pci 00:00.0 bridge to [bus a-b]
    pci a:01.0 bridge to [bus c-d] (already enumerated)
    pci a:02.0 bridge to [bus e-f] (already enumerated)
    pci a:03.0 bridge to [bus x-y] (enumerating now)

    We know [c-d] is contained in [a-b]; [e-f] is contained in [a-b]; a <
    c; and a < e. To enumerate behind a:03.0, we need to assign x & y
    such that a < x; [x-y] is contained in [a-b]; and [x-y] does not
    overlap [c-d] or [e-f]. The value from pci_scan_child_bus() is
    probably useful for setting y, but we don't have to save it in the
    struct pci_bus for that.

    > and also we have one resource to insert it into the resource tree, so
    > later could probe/allocate bus num range.

    Sorry, I didn't understand this.

    Bjorn
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