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
(dev->priv) then it is up to the module exit handler to free that.


struct net_device synchronization rules
=======================================
dev->open:
	Synchronization: rtnl_lock() semaphore.
	Context: process

dev->stop:
	Synchronization: rtnl_lock() semaphore.
	Context: process
	Note1: netif_running() is guaranteed false
	Note2: dev->poll() is guaranteed to be stopped

dev->do_ioctl:
	Synchronization: rtnl_lock() semaphore.
	Context: process

dev->get_stats:
	Synchronization: dev_base_lock rwlock.
	Context: nominally process, but don't sleep inside an rwlock

dev->hard_start_xmit:
	Synchronization: dev->xmit_lock spinlock.
	When the driver sets NETIF_F_LLTX in dev->features this will be
	called without holding xmit_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 -1 when the spin lock fails. 
	The locking there should also properly protect against 
	set_multicast_list
	Context: BHs disabled
	Notes: netif_queue_stopped() is guaranteed false
               Interrupts must be enabled when calling hard_start_xmit.
                (Interrupts must also be enabled when enabling the BH handler.)
	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.

dev->tx_timeout:
	Synchronization: dev->xmit_lock spinlock.
	Context: BHs disabled
	Notes: netif_queue_stopped() is guaranteed true

dev->set_multicast_list:
	Synchronization: dev->xmit_lock spinlock.
	Context: BHs disabled

dev->poll:
	Synchronization: __LINK_STATE_RX_SCHED bit in dev->state.  See
		dev_close code and comments in net/core/dev.c for more info.
	Context: softirq