On the driver side, pci_register_driver() is called when a driver module is loaded, or at boot time if the module is built-in. (Whenever a device/driver is added, driver/device list is looped to find a match, I get that part.)
But where/when are pci devices discovered and registered with the bus? I imagine this is arch specific, and would involve BIOS on x86, such as - BIOS routine probe PCI devices and then put the results in a list some where in RAM, before loading the kernel, and each list entry contains information of a single pci device including vendorId/deviceId etc. Kernel then pick up the list and insert them into pci_bus_type.devices at some point. But this is pure guess, can anyone give some hints?