1.. SPDX-License-Identifier: GPL-2.0 2 3===================================== 4Network Devices, the Kernel, and You! 5===================================== 6 7 8Introduction 9============ 10The following is a random collection of documentation regarding 11network devices. 12 13struct net_device allocation rules 14================================== 15Network device structures need to persist even after module is unloaded and 16must be allocated with alloc_netdev_mqs() and friends. 17If device has registered successfully, it will be freed on last use 18by free_netdev(). This is required to handle the pathologic case cleanly 19(example: rmmod mydriver </sys/class/net/myeth/mtu ) 20 21alloc_netdev_mqs()/alloc_netdev() reserve extra space for driver 22private data which gets freed when the network device is freed. If 23separately allocated data is attached to the network device 24(netdev_priv(dev)) then it is up to the module exit handler to free that. 25 26MTU 27=== 28Each network device has a Maximum Transfer Unit. The MTU does not 29include any link layer protocol overhead. Upper layer protocols must 30not pass a socket buffer (skb) to a device to transmit with more data 31than the mtu. The MTU does not include link layer header overhead, so 32for example on Ethernet if the standard MTU is 1500 bytes used, the 33actual skb will contain up to 1514 bytes because of the Ethernet 34header. Devices should allow for the 4 byte VLAN header as well. 35 36Segmentation Offload (GSO, TSO) is an exception to this rule. The 37upper layer protocol may pass a large socket buffer to the device 38transmit routine, and the device will break that up into separate 39packets based on the current MTU. 40 41MTU is symmetrical and applies both to receive and transmit. A device 42must be able to receive at least the maximum size packet allowed by 43the MTU. A network device may use the MTU as mechanism to size receive 44buffers, but the device should allow packets with VLAN header. With 45standard Ethernet mtu of 1500 bytes, the device should allow up to 461518 byte packets (1500 + 14 header + 4 tag). The device may either: 47drop, truncate, or pass up oversize packets, but dropping oversize 48packets is preferred. 49 50 51struct net_device synchronization rules 52======================================= 53ndo_open: 54 Synchronization: rtnl_lock() semaphore. 55 Context: process 56 57ndo_stop: 58 Synchronization: rtnl_lock() semaphore. 59 Context: process 60 Note: netif_running() is guaranteed false 61 62ndo_do_ioctl: 63 Synchronization: rtnl_lock() semaphore. 64 Context: process 65 66ndo_get_stats: 67 Synchronization: dev_base_lock rwlock. 68 Context: nominally process, but don't sleep inside an rwlock 69 70ndo_start_xmit: 71 Synchronization: __netif_tx_lock spinlock. 72 73 When the driver sets NETIF_F_LLTX in dev->features this will be 74 called without holding netif_tx_lock. In this case the driver 75 has to lock by itself when needed. 76 The locking there should also properly protect against 77 set_rx_mode. WARNING: use of NETIF_F_LLTX is deprecated. 78 Don't use it for new drivers. 79 80 Context: Process with BHs disabled or BH (timer), 81 will be called with interrupts disabled by netconsole. 82 83 Return codes: 84 85 * NETDEV_TX_OK everything ok. 86 * NETDEV_TX_BUSY Cannot transmit packet, try later 87 Usually a bug, means queue start/stop flow control is broken in 88 the driver. Note: the driver must NOT put the skb in its DMA ring. 89 90ndo_tx_timeout: 91 Synchronization: netif_tx_lock spinlock; all TX queues frozen. 92 Context: BHs disabled 93 Notes: netif_queue_stopped() is guaranteed true 94 95ndo_set_rx_mode: 96 Synchronization: netif_addr_lock spinlock. 97 Context: BHs disabled 98 99struct napi_struct synchronization rules 100======================================== 101napi->poll: 102 Synchronization: 103 NAPI_STATE_SCHED bit in napi->state. Device 104 driver's ndo_stop method will invoke napi_disable() on 105 all NAPI instances which will do a sleeping poll on the 106 NAPI_STATE_SCHED napi->state bit, waiting for all pending 107 NAPI activity to cease. 108 109 Context: 110 softirq 111 will be called with interrupts disabled by netconsole. 112