On Sun, Apr 1, 2012 at 2:38 AM, Richard Yang <weiyang@xxxxxxxxxxxxxxxxxx> 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. 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 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. 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. Bjorn -- 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
