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Linus Torvalds1da177e2005-04-16 15:20:36 -07001
2Network Devices, the Kernel, and You!
3
4
5Introduction
6============
7The following is a random collection of documentation regarding
8network devices.
9
10struct net_device allocation rules
11==================================
12Network device structures need to persist even after module is unloaded and
13must be allocated with kmalloc. If device has registered successfully,
14it will be freed on last use by free_netdev. This is required to handle the
15pathologic case cleanly (example: rmmod mydriver </sys/class/net/myeth/mtu )
16
17There are routines in net_init.c to handle the common cases of
18alloc_etherdev, alloc_netdev. These reserve extra space for driver
19private data which gets freed when the network device is freed. If
20separately allocated data is attached to the network device
Wang Chenb74ca3a2008-12-08 01:14:16 -080021(netdev_priv(dev)) then it is up to the module exit handler to free that.
Linus Torvalds1da177e2005-04-16 15:20:36 -070022
Stephen Hemminger1c8c7d62007-07-07 23:03:44 -070023MTU
24===
25Each network device has a Maximum Transfer Unit. The MTU does not
26include any link layer protocol overhead. Upper layer protocols must
27not pass a socket buffer (skb) to a device to transmit with more data
28than the mtu. The MTU does not include link layer header overhead, so
29for example on Ethernet if the standard MTU is 1500 bytes used, the
30actual skb will contain up to 1514 bytes because of the Ethernet
31header. Devices should allow for the 4 byte VLAN header as well.
32
33Segmentation Offload (GSO, TSO) is an exception to this rule. The
34upper layer protocol may pass a large socket buffer to the device
35transmit routine, and the device will break that up into separate
36packets based on the current MTU.
37
38MTU is symmetrical and applies both to receive and transmit. A device
39must be able to receive at least the maximum size packet allowed by
40the MTU. A network device may use the MTU as mechanism to size receive
41buffers, but the device should allow packets with VLAN header. With
42standard Ethernet mtu of 1500 bytes, the device should allow up to
431518 byte packets (1500 + 14 header + 4 tag). The device may either:
44drop, truncate, or pass up oversize packets, but dropping oversize
45packets is preferred.
46
47
Linus Torvalds1da177e2005-04-16 15:20:36 -070048struct net_device synchronization rules
49=======================================
50dev->open:
51 Synchronization: rtnl_lock() semaphore.
52 Context: process
53
54dev->stop:
55 Synchronization: rtnl_lock() semaphore.
56 Context: process
57 Note1: netif_running() is guaranteed false
58 Note2: dev->poll() is guaranteed to be stopped
59
60dev->do_ioctl:
61 Synchronization: rtnl_lock() semaphore.
62 Context: process
63
64dev->get_stats:
65 Synchronization: dev_base_lock rwlock.
66 Context: nominally process, but don't sleep inside an rwlock
67
68dev->hard_start_xmit:
Herbert Xu932ff272006-06-09 12:20:56 -070069 Synchronization: netif_tx_lock spinlock.
Stephen Hemminger17229332007-07-07 22:59:14 -070070
Linus Torvalds1da177e2005-04-16 15:20:36 -070071 When the driver sets NETIF_F_LLTX in dev->features this will be
Herbert Xu932ff272006-06-09 12:20:56 -070072 called without holding netif_tx_lock. In this case the driver
Linus Torvalds1da177e2005-04-16 15:20:36 -070073 has to lock by itself when needed. It is recommended to use a try lock
Stephen Hemminger17229332007-07-07 22:59:14 -070074 for this and return NETDEV_TX_LOCKED when the spin lock fails.
Linus Torvalds1da177e2005-04-16 15:20:36 -070075 The locking there should also properly protect against
Christian Borntraegere24eb522007-09-25 19:42:02 -070076 set_multicast_list. Note that the use of NETIF_F_LLTX is deprecated.
Matt LaPlante19f59462009-04-27 15:06:31 +020077 Don't use it for new drivers.
Stephen Hemminger17229332007-07-07 22:59:14 -070078
79 Context: Process with BHs disabled or BH (timer),
80 will be called with interrupts disabled by netconsole.
81
Linus Torvalds1da177e2005-04-16 15:20:36 -070082 Return codes:
83 o NETDEV_TX_OK everything ok.
84 o NETDEV_TX_BUSY Cannot transmit packet, try later
85 Usually a bug, means queue start/stop flow control is broken in
86 the driver. Note: the driver must NOT put the skb in its DMA ring.
87 o NETDEV_TX_LOCKED Locking failed, please retry quickly.
88 Only valid when NETIF_F_LLTX is set.
89
90dev->tx_timeout:
Herbert Xu932ff272006-06-09 12:20:56 -070091 Synchronization: netif_tx_lock spinlock.
Linus Torvalds1da177e2005-04-16 15:20:36 -070092 Context: BHs disabled
93 Notes: netif_queue_stopped() is guaranteed true
94
95dev->set_multicast_list:
Herbert Xu932ff272006-06-09 12:20:56 -070096 Synchronization: netif_tx_lock spinlock.
Linus Torvalds1da177e2005-04-16 15:20:36 -070097 Context: BHs disabled
98
Stephen Hemmingerbea33482007-10-03 16:41:36 -070099struct napi_struct synchronization rules
100========================================
101napi->poll:
102 Synchronization: NAPI_STATE_SCHED bit in napi->state. Device
103 driver's dev->close method will invoke napi_disable() on
104 all NAPI instances which will do a sleeping poll on the
105 NAPI_STATE_SCHED napi->state bit, waiting for all pending
106 NAPI activity to cease.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700107 Context: softirq
Stephen Hemminger17229332007-07-07 22:59:14 -0700108 will be called with interrupts disabled by netconsole.