Introduction¶

The following is a random collection of documentation regardingnetwork devices.

struct net_device allocation rules¶

Network device structures need to persist even after module is unloaded andmust be allocated withalloc_netdev_mqs() and friends.If device has registered successfully, it will be freed on last usebyfree_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 driverprivate data which gets freed when the network device is freed. Ifseparately 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 notinclude any link layer protocol overhead. Upper layer protocols mustnot pass a socket buffer (skb) to a device to transmit with more datathan the mtu. The MTU does not include link layer header overhead, sofor example on Ethernet if the standard MTU is 1500 bytes used, theactual skb will contain up to 1514 bytes because of the Ethernetheader. Devices should allow for the 4 byte VLAN header as well.

Segmentation Offload (GSO, TSO) is an exception to this rule. Theupper layer protocol may pass a large socket buffer to the devicetransmit routine, and the device will break that up into separatepackets based on the current MTU.

MTU is symmetrical and applies both to receive and transmit. A devicemust be able to receive at least the maximum size packet allowed bythe MTU. A network device may use the MTU as mechanism to size receivebuffers, but the device should allow packets with VLAN header. Withstandard Ethernet mtu of 1500 bytes, the device should allow up to1518 byte packets (1500 + 14 header + 4 tag). The device may either:drop, truncate, or pass up oversize packets, but dropping oversizepackets is preferred.

struct net_device synchronization rules¶

ndo_open:

Synchronization: rtnl_lock() semaphore.Context: process

ndo_stop:

Synchronization: rtnl_lock() semaphore.Context: processNote: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 becalled without holding netif_tx_lock. In this case the driverhas to lock by itself when needed.The locking there should also properly protect againstset_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 laterUsually a bug, means queue start/stop flow control is broken inthe 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 disabledNotes: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. Devicedriver’s ndo_stop method will invokenapi_disable() onall NAPI instances which will do a sleeping poll on theNAPI_STATE_SCHED napi->state bit, waiting for all pendingNAPI activity to cease.

Context:

softirqwill be called with interrupts disabled by netconsole.