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+
+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 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.
+
+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. 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.
+
+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.