Re: What is the difference between number and secondary in pci_bus?

[Richard Yang <weiyang@linux.vnet.ibm.com>]

On Mon, Apr 02, 2012 at 05:30:52PM -0600, Bjorn Helgaas wrote:
>On Sun, Apr 1, 2012 at 2:38 AM, Richard Yang <weiyang@linux.vnet.ibm.com> wrote:
>> All
>>
>> In the structure of pci_bus.
>> There is number, primary, secondary and subordinate.
>>
>> primary and subordinate are easy to understand, while the number and
>> secondary confused me.
>>
>> I think secondary means the current bus number.
>> Then what number stands for?
>>
>> In pci_alloc_child_bus(),
>> child->number = child->secondary = busnr;
>> So they are the same?
>> Then we need two different field?
>
>Good questions.
Thanks for your response.
>
>Primary, secondary, and subordinate are really properties of the
>upstream bridge, not the bus itself.  If we had a struct pci_bridge,
>they would logically live there.  Primary is the bus number of the
>upstream side of the bridge.  Secondary is the bus number of the
>bridge's downstream side.  The bridge forwards config transactions
>with bus numbers between secondary and subordinate to its downstream
>side.
>
>The "number of primary/secondary bridge" comments in struct pci_bus
>are wrong -- they are bus numbers, not bridge numbers.  And the "max
So do you think it is better to change "bridge" to "number"?
>number of subordinate buses" comment is not really right either.
>Subordinate is just the largest bus number forwarded by the bridge.
>As a consequence, that limits the number of buses in the downstream
>hierarchy.
Yes, sounds it is not very exact. Or just "subordinate bus" is ok?
>
>I have no idea why we have both number and secondary in struct
>pci_bus.  It seems like they should always be the same.  It would be
>an interesting exercise to figure out if they're ever different.
>
Do you think add some printk() at the end of pci_scan_bridge() is a good
place to test this?
If they are the same, could we remove one?

BTW, I tried git blame to search who add these two fields. Seems those
code is imported by 1da177e, which is the start of the git system.
>Bjorn

-- 
Richard Yang
Help you, Help me