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-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 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.
-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. 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.
- 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
- 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.