Re: [PATCH -v11 04/30] PCI: Add busn_res into struct pci_bus.

[Bjorn Helgaas <bhelgaas@google.com>]

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