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Documentation / networking / netdevices.txt




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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.

1	
2	Network Devices, the Kernel, and You!
3	
4	
5	Introduction
6	============
7	The following is a random collection of documentation regarding
8	network devices.
9	
10	struct net_device allocation rules
11	==================================
12	Network device structures need to persist even after module is unloaded and
13	must be allocated with alloc_netdev_mqs() and friends.
14	If device has registered successfully, it will be freed on last use
15	by free_netdev(). This is required to handle the pathologic case cleanly
16	(example: rmmod mydriver </sys/class/net/myeth/mtu )
17	
18	alloc_netdev_mqs()/alloc_netdev() reserve extra space for driver
19	private data which gets freed when the network device is freed. If
20	separately allocated data is attached to the network device
21	(netdev_priv(dev)) then it is up to the module exit handler to free that.
22	
23	MTU
24	===
25	Each network device has a Maximum Transfer Unit. The MTU does not
26	include any link layer protocol overhead. Upper layer protocols must
27	not pass a socket buffer (skb) to a device to transmit with more data
28	than the mtu. The MTU does not include link layer header overhead, so
29	for example on Ethernet if the standard MTU is 1500 bytes used, the
30	actual skb will contain up to 1514 bytes because of the Ethernet
31	header. Devices should allow for the 4 byte VLAN header as well.
32	
33	Segmentation Offload (GSO, TSO) is an exception to this rule.  The
34	upper layer protocol may pass a large socket buffer to the device
35	transmit routine, and the device will break that up into separate
36	packets based on the current MTU.
37	
38	MTU is symmetrical and applies both to receive and transmit. A device
39	must be able to receive at least the maximum size packet allowed by
40	the MTU. A network device may use the MTU as mechanism to size receive
41	buffers, but the device should allow packets with VLAN header. With
42	standard Ethernet mtu of 1500 bytes, the device should allow up to
43	1518 byte packets (1500 + 14 header + 4 tag).  The device may either:
44	drop, truncate, or pass up oversize packets, but dropping oversize
45	packets is preferred.
46	
47	
48	struct net_device synchronization rules
49	=======================================
50	ndo_open:
51		Synchronization: rtnl_lock() semaphore.
52		Context: process
53	
54	ndo_stop:
55		Synchronization: rtnl_lock() semaphore.
56		Context: process
57		Note: netif_running() is guaranteed false
58	
59	ndo_do_ioctl:
60		Synchronization: rtnl_lock() semaphore.
61		Context: process
62	
63	ndo_get_stats:
64		Synchronization: dev_base_lock rwlock.
65		Context: nominally process, but don't sleep inside an rwlock
66	
67	ndo_start_xmit:
68		Synchronization: __netif_tx_lock spinlock.
69	
70		When the driver sets NETIF_F_LLTX in dev->features this will be
71		called without holding netif_tx_lock. In this case the driver
72		has to lock by itself when needed. It is recommended to use a try lock
73		for this and return NETDEV_TX_LOCKED when the spin lock fails.
74		The locking there should also properly protect against 
75		set_rx_mode. Note that the use of NETIF_F_LLTX is deprecated.
76		Don't use it for new drivers.
77	
78		Context: Process with BHs disabled or BH (timer),
79		         will be called with interrupts disabled by netconsole.
80	
81		Return codes: 
82		o NETDEV_TX_OK everything ok. 
83		o NETDEV_TX_BUSY Cannot transmit packet, try later 
84		  Usually a bug, means queue start/stop flow control is broken in
85		  the driver. Note: the driver must NOT put the skb in its DMA ring.
86		o NETDEV_TX_LOCKED Locking failed, please retry quickly.
87		  Only valid when NETIF_F_LLTX is set.
88	
89	ndo_tx_timeout:
90		Synchronization: netif_tx_lock spinlock; all TX queues frozen.
91		Context: BHs disabled
92		Notes: netif_queue_stopped() is guaranteed true
93	
94	ndo_set_rx_mode:
95		Synchronization: netif_addr_lock spinlock.
96		Context: BHs disabled
97	
98	struct napi_struct synchronization rules
99	========================================
100	napi->poll:
101		Synchronization: NAPI_STATE_SCHED bit in napi->state.  Device
102			driver's ndo_stop method will invoke napi_disable() on
103			all NAPI instances which will do a sleeping poll on the
104			NAPI_STATE_SCHED napi->state bit, waiting for all pending
105			NAPI activity to cease.
106		Context: softirq
107		         will be called with interrupts disabled by netconsole.
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