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 alloc_netdev_mqs() and friends.
  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 )
  
  alloc_netdev_mqs()/alloc_netdev() 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
  =======================================
  ndo_open:
  	Synchronization: rtnl_lock() semaphore.
  	Context: process
  
  ndo_stop:
  	Synchronization: rtnl_lock() semaphore.
  	Context: process
  	Note: netif_running() is guaranteed false
  
  ndo_do_ioctl:
  	Synchronization: rtnl_lock() semaphore.
  	Context: process
  
  ndo_get_stats:
  	Synchronization: dev_base_lock rwlock.
  	Context: nominally process, but don't sleep inside an rwlock
  
  ndo_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.
  	The locking there should also properly protect against
  	set_rx_mode. WARNING: 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.
  
  ndo_tx_timeout:
  	Synchronization: netif_tx_lock spinlock; all TX queues frozen.
  	Context: BHs disabled
  	Notes: netif_queue_stopped() is guaranteed true
  
  ndo_set_rx_mode:
  	Synchronization: netif_addr_lock spinlock.
  	Context: BHs disabled
  
  struct napi_struct synchronization rules
  ========================================
  napi->poll:
  	Synchronization: NAPI_STATE_SCHED bit in napi->state.  Device
  		driver's ndo_stop 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.