====== gPXE Network Driver Initialization ======
What happens when gPXE starts up?\\
How does a network driver get initialized?\\

Let's follow the call stack at key points from instantiation.\\
===== Reaching probe =====
  * src/core/main.c >> ''main()'' -- Entry point.
  * src/core/init.c >> ''startup()''
  * src/core/device.c >> ''probe_devices()'' -- Probe buses sequentially.
  * src/core/device.c >> ''rootdev_probe()'' -- Probe this bus.
  * ''%%{struct root_device}->driver->probe()%%'' == src/drivers/bus/pci.c >> ''pcibus_probe()'' -- Probe PCI devices sequentially.
  * src/drivers/bus/pci.c >> ''pci_probe()'' -- Probe this device.
  * ''%%{struct pci_driver}->probe()%%'' == src/drivers/net/NAME.c >> ''NAME_probe()'' -- The network driver's ''probe()'' routine is finally reached.
At this point ''netdev_init()'' associates netdev with driver functions.\\ 
''register_netdev()'' adds to the device list.\\ 
We return up the call stack to main().
===== Reaching close =====
  * src/core/main.c >> ''main()''
  * src/usr/autoboot.c >> ''autoboot()'' -- Boot network devices sequentially.
  * src/usr/autoboot.c >> ''close_all_netdevs()''
  * src/usr/ifmgmt.c >> ''ifclose()''
  * src/net/netdevice.c >> ''netdev_close()''
  * ''%%{struct net_device}->op->close()%%'' == src/drivers/net/NAME.c >> ''NAME_close()'' -- The network driver's ''close()'' routine is reached.
All network devices are closed before attempting to boot each network device.\\ 
We return up the call stack to autoboot().
===== Reaching open =====
  * src/usr/autoboot.c >> ''autoboot()'' -- Still booting network devices sequentially.
  * src/usr/autoboot.c >> ''netboot()'' -- Boot this network device
  * src/usr/ifmgmt.c >> ''ifopen()''
  * src/net/netdevice.c >> ''netdev_open()''
  * ''%%{struct net_device}->op->open()%%'' == src/drivers/net/NAME.c >> ''NAME_open()'' -- The network driver's ''open()'' routine is reached.
At this point transmit and receive are enabled.

===== root_device and pci_device structs =====
Both ''struct root_device'' and ''struct pci_device'' have a special function in gPXE.  Bus drivers are ''root_device''s, thus there is one for PCI, ISA, etc.  PCI devices are self-explanatory.  Both ''root_device'' and ''pci_device'' structs are defined within their respective driver code.  These structs hold function pointers to essential interface routines, such as ''probe()'' and ''remove()''.  The ''%%__pci_driver%%'' and ''%%__root_driver%%'' tags cause the drivers to be added to a list of modules, resulting in their ''probe()'' routines being called during initialization.