linux/Documentation/networking/netdevices.txt
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   2Network Devices, the Kernel, and You!
   3
   4
   5Introduction
   6============
   7The following is a random collection of documentation regarding
   8network devices.
   9
  10struct net_device allocation rules
  11==================================
  12Network device structures need to persist even after module is unloaded and
  13must be allocated with kmalloc.  If device has registered successfully,
  14it will be freed on last use by free_netdev.  This is required to handle the
  15pathologic case cleanly (example: rmmod mydriver </sys/class/net/myeth/mtu )
  16
  17There are routines in net_init.c to handle the common cases of
  18alloc_etherdev, alloc_netdev.  These reserve extra space for driver
  19private data which gets freed when the network device is freed. If
  20separately allocated data is attached to the network device
  21(netdev_priv(dev)) then it is up to the module exit handler to free that.
  22
  23MTU
  24===
  25Each network device has a Maximum Transfer Unit. The MTU does not
  26include any link layer protocol overhead. Upper layer protocols must
  27not pass a socket buffer (skb) to a device to transmit with more data
  28than the mtu. The MTU does not include link layer header overhead, so
  29for example on Ethernet if the standard MTU is 1500 bytes used, the
  30actual skb will contain up to 1514 bytes because of the Ethernet
  31header. Devices should allow for the 4 byte VLAN header as well.
  32
  33Segmentation Offload (GSO, TSO) is an exception to this rule.  The
  34upper layer protocol may pass a large socket buffer to the device
  35transmit routine, and the device will break that up into separate
  36packets based on the current MTU.
  37
  38MTU is symmetrical and applies both to receive and transmit. A device
  39must be able to receive at least the maximum size packet allowed by
  40the MTU. A network device may use the MTU as mechanism to size receive
  41buffers, but the device should allow packets with VLAN header. With
  42standard Ethernet mtu of 1500 bytes, the device should allow up to
  431518 byte packets (1500 + 14 header + 4 tag).  The device may either:
  44drop, truncate, or pass up oversize packets, but dropping oversize
  45packets is preferred.
  46
  47
  48struct net_device synchronization rules
  49=======================================
  50dev->open:
  51        Synchronization: rtnl_lock() semaphore.
  52        Context: process
  53
  54dev->stop:
  55        Synchronization: rtnl_lock() semaphore.
  56        Context: process
  57        Note1: netif_running() is guaranteed false
  58        Note2: dev->poll() is guaranteed to be stopped
  59
  60dev->do_ioctl:
  61        Synchronization: rtnl_lock() semaphore.
  62        Context: process
  63
  64dev->get_stats:
  65        Synchronization: dev_base_lock rwlock.
  66        Context: nominally process, but don't sleep inside an rwlock
  67
  68dev->hard_start_xmit:
  69        Synchronization: netif_tx_lock spinlock.
  70
  71        When the driver sets NETIF_F_LLTX in dev->features this will be
  72        called without holding netif_tx_lock. In this case the driver
  73        has to lock by itself when needed. It is recommended to use a try lock
  74        for this and return NETDEV_TX_LOCKED when the spin lock fails.
  75        The locking there should also properly protect against 
  76        set_multicast_list. Note that the use of NETIF_F_LLTX is deprecated.
  77        Dont use it for new drivers.
  78
  79        Context: Process with BHs disabled or BH (timer),
  80                 will be called with interrupts disabled by netconsole.
  81
  82        Return codes: 
  83        o NETDEV_TX_OK everything ok. 
  84        o NETDEV_TX_BUSY Cannot transmit packet, try later 
  85          Usually a bug, means queue start/stop flow control is broken in
  86          the driver. Note: the driver must NOT put the skb in its DMA ring.
  87        o NETDEV_TX_LOCKED Locking failed, please retry quickly.
  88          Only valid when NETIF_F_LLTX is set.
  89
  90dev->tx_timeout:
  91        Synchronization: netif_tx_lock spinlock.
  92        Context: BHs disabled
  93        Notes: netif_queue_stopped() is guaranteed true
  94
  95dev->set_multicast_list:
  96        Synchronization: netif_tx_lock spinlock.
  97        Context: BHs disabled
  98
  99struct napi_struct synchronization rules
 100========================================
 101napi->poll:
 102        Synchronization: NAPI_STATE_SCHED bit in napi->state.  Device
 103                driver's dev->close method will invoke napi_disable() on
 104                all NAPI instances which will do a sleeping poll on the
 105                NAPI_STATE_SCHED napi->state bit, waiting for all pending
 106                NAPI activity to cease.
 107        Context: softirq
 108                 will be called with interrupts disabled by netconsole.
 109
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