Network Devices, the Kernel, and You!


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.


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

Synchronization: rtnl_lock() semaphore. Context: process
Synchronization: rtnl_lock() semaphore. Context: process Note: netif_running() is guaranteed false
Synchronization: rtnl_lock() semaphore. Context: process
Synchronization: dev_base_lock rwlock. Context: nominally process, but don’t sleep inside an rwlock

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:

  • NETDEV_TX_OK everything ok.
  • 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.
Synchronization: netif_tx_lock spinlock; all TX queues frozen. Context: BHs disabled Notes: netif_queue_stopped() is guaranteed true
Synchronization: netif_addr_lock spinlock. Context: BHs disabled

struct napi_struct synchronization rules

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.
softirq will be called with interrupts disabled by netconsole.