Understanding Linux Network Internals

The Big Picture

Let's put together what we saw in the previous sections and see what happens at boot time in a system with a PCI bus and a few PCI devices.^[*]

When the system boots, it creates a sort of database that associates each bus to a list of detected devices that use the bus. For example, the descriptor for the PCI bus includes, among other parameters, a list of detected PCI devices. As we saw in the section "Registering a PCI NIC Device Driver," each PCI device is uniquely identified by a large collection of fields in the structure pci_device_id, although only a few are usually necessary. We also saw how PCI device drivers define an instance of pci_driver and register with the PCI layer with pci_register_driver (or its alias, pci_module_init). By the time device drivers are loaded, the kernel has already built its database:^[†] let's then take the example of Figure 6-1(a) with three PCI devices and see what happens when device drivers A and B are loaded.

When device driver A is loaded, it registers with the PCI layer by calling pci_register_driver and providing its instance of pci_driver. The pci_driver structure includes a vector with the IDs of those PCI devices it can drive. The PCI layer then uses that table to see what devices match in its list of detected PCI devices. It thus creates the driver's device list shown in Figure 6-1(b). In addition, for each matching device, the PCI layer invokes the probe function provided by the matching driver in its pci_driver structure. ...