Network Devices, the Kernel, and You!
  
  
  Introduction
  ============
  The following is a random collection of documentation regarding
  network devices.
  
  struct net_device allocation rules
  ==================================
  Network device structures need to persist even after module is unloaded and
  must be allocated with kmalloc.  If device has registered successfully,
  it will be freed on last use by free_netdev.  This is required to handle the
  pathologic case cleanly (example: rmmod mydriver </sys/class/net/myeth/mtu )
  
  There are routines in net_init.c to handle the common cases of
  alloc_etherdev, alloc_netdev.  These reserve extra space for driver
  private data which gets freed when the network device is freed. If
  separately allocated data is attached to the network device

  (netdev_priv(dev)) then it is up to the module exit handler to free that.

  MTU
  ===
  Each network device has a Maximum Transfer Unit. The MTU does not
  include any link layer protocol overhead. Upper layer protocols must
  not pass a socket buffer (skb) to a device to transmit with more data
  than the mtu. The MTU does not include link layer header overhead, so
  for example on Ethernet if the standard MTU is 1500 bytes used, the
  actual skb will contain up to 1514 bytes because of the Ethernet
  header. Devices should allow for the 4 byte VLAN header as well.
  
  Segmentation Offload (GSO, TSO) is an exception to this rule.  The
  upper layer protocol may pass a large socket buffer to the device
  transmit routine, and the device will break that up into separate
  packets based on the current MTU.
  
  MTU is symmetrical and applies both to receive and transmit. A device
  must be able to receive at least the maximum size packet allowed by
  the MTU. A network device may use the MTU as mechanism to size receive
  buffers, but the device should allow packets with VLAN header. With
  standard Ethernet mtu of 1500 bytes, the device should allow up to
  1518 byte packets (1500 + 14 header + 4 tag).  The device may either:
  drop, truncate, or pass up oversize packets, but dropping oversize
  packets is preferred.

  struct net_device synchronization rules
  =======================================
  dev->open:
  	Synchronization: rtnl_lock() semaphore.
  	Context: process
  
  dev->stop:
  	Synchronization: rtnl_lock() semaphore.
  	Context: process
  	Note1: netif_running() is guaranteed false
  	Note2: dev->poll() is guaranteed to be stopped
  
  dev->do_ioctl:
  	Synchronization: rtnl_lock() semaphore.
  	Context: process
  
  dev->get_stats:
  	Synchronization: dev_base_lock rwlock.
  	Context: nominally process, but don't sleep inside an rwlock
  
  dev->hard_start_xmit:

  	Synchronization: netif_tx_lock spinlock.

  	When the driver sets NETIF_F_LLTX in dev->features this will be

  	called without holding netif_tx_lock. In this case the driver

  	has to lock by itself when needed. It is recommended to use a try lock

  	for this and return NETDEV_TX_LOCKED when the spin lock fails.

  	The locking there should also properly protect against 

  	set_rx_mode. Note that the use of NETIF_F_LLTX is deprecated.

  	Don't use it for new drivers.

  
  	Context: Process with BHs disabled or BH (timer),
  	         will be called with interrupts disabled by netconsole.

  	Return codes: 
  	o NETDEV_TX_OK everything ok. 
  	o NETDEV_TX_BUSY Cannot transmit packet, try later 
  	  Usually a bug, means queue start/stop flow control is broken in
  	  the driver. Note: the driver must NOT put the skb in its DMA ring.
  	o NETDEV_TX_LOCKED Locking failed, please retry quickly.
  	  Only valid when NETIF_F_LLTX is set.
  
  dev->tx_timeout:

  	Synchronization: netif_tx_lock spinlock.

  	Context: BHs disabled
  	Notes: netif_queue_stopped() is guaranteed true

  dev->set_rx_mode:

  	Synchronization: netif_tx_lock spinlock.

  	Context: BHs disabled

  struct napi_struct synchronization rules
  ========================================
  napi->poll:
  	Synchronization: NAPI_STATE_SCHED bit in napi->state.  Device
  		driver's dev->close method will invoke napi_disable() on
  		all NAPI instances which will do a sleeping poll on the
  		NAPI_STATE_SCHED napi->state bit, waiting for all pending
  		NAPI activity to cease.

  	Context: softirq

  	         will be called with interrupts disabled by netconsole.