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107 lines
4.0 KiB
107 lines
4.0 KiB
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Network Devices, the Kernel, and You! |
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Introduction |
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============ |
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The following is a random collection of documentation regarding |
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network devices. |
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struct net_device allocation rules |
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================================== |
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Network device structures need to persist even after module is unloaded and |
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must be allocated with alloc_netdev_mqs() and friends. |
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If device has registered successfully, it will be freed on last use |
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by free_netdev(). This is required to handle the pathologic case cleanly |
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(example: rmmod mydriver </sys/class/net/myeth/mtu ) |
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alloc_netdev_mqs()/alloc_netdev() reserve extra space for driver |
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private data which gets freed when the network device is freed. If |
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separately allocated data is attached to the network device |
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(netdev_priv(dev)) then it is up to the module exit handler to free that. |
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MTU |
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=== |
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Each network device has a Maximum Transfer Unit. The MTU does not |
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include any link layer protocol overhead. Upper layer protocols must |
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not pass a socket buffer (skb) to a device to transmit with more data |
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than the mtu. The MTU does not include link layer header overhead, so |
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for example on Ethernet if the standard MTU is 1500 bytes used, the |
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actual skb will contain up to 1514 bytes because of the Ethernet |
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header. Devices should allow for the 4 byte VLAN header as well. |
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Segmentation Offload (GSO, TSO) is an exception to this rule. The |
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upper layer protocol may pass a large socket buffer to the device |
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transmit routine, and the device will break that up into separate |
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packets based on the current MTU. |
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MTU is symmetrical and applies both to receive and transmit. A device |
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must be able to receive at least the maximum size packet allowed by |
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the MTU. A network device may use the MTU as mechanism to size receive |
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buffers, but the device should allow packets with VLAN header. With |
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standard Ethernet mtu of 1500 bytes, the device should allow up to |
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1518 byte packets (1500 + 14 header + 4 tag). The device may either: |
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drop, truncate, or pass up oversize packets, but dropping oversize |
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packets is preferred. |
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struct net_device synchronization rules |
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======================================= |
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ndo_open: |
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Synchronization: rtnl_lock() semaphore. |
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Context: process |
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ndo_stop: |
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Synchronization: rtnl_lock() semaphore. |
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Context: process |
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Note: netif_running() is guaranteed false |
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ndo_do_ioctl: |
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Synchronization: rtnl_lock() semaphore. |
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Context: process |
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ndo_get_stats: |
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Synchronization: dev_base_lock rwlock. |
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Context: nominally process, but don't sleep inside an rwlock |
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ndo_start_xmit: |
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Synchronization: __netif_tx_lock spinlock. |
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When the driver sets NETIF_F_LLTX in dev->features this will be |
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called without holding netif_tx_lock. In this case the driver |
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has to lock by itself when needed. It is recommended to use a try lock |
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for this and return NETDEV_TX_LOCKED when the spin lock fails. |
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The locking there should also properly protect against |
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set_rx_mode. Note that the use of NETIF_F_LLTX is deprecated. |
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Don't use it for new drivers. |
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Context: Process with BHs disabled or BH (timer), |
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will be called with interrupts disabled by netconsole. |
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Return codes: |
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o NETDEV_TX_OK everything ok. |
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o NETDEV_TX_BUSY Cannot transmit packet, try later |
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Usually a bug, means queue start/stop flow control is broken in |
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the driver. Note: the driver must NOT put the skb in its DMA ring. |
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o NETDEV_TX_LOCKED Locking failed, please retry quickly. |
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Only valid when NETIF_F_LLTX is set. |
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ndo_tx_timeout: |
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Synchronization: netif_tx_lock spinlock; all TX queues frozen. |
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Context: BHs disabled |
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Notes: netif_queue_stopped() is guaranteed true |
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ndo_set_rx_mode: |
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Synchronization: netif_addr_lock spinlock. |
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Context: BHs disabled |
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struct napi_struct synchronization rules |
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======================================== |
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napi->poll: |
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Synchronization: NAPI_STATE_SCHED bit in napi->state. Device |
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driver's ndo_stop method will invoke napi_disable() on |
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all NAPI instances which will do a sleeping poll on the |
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NAPI_STATE_SCHED napi->state bit, waiting for all pending |
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NAPI activity to cease. |
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Context: softirq |
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will be called with interrupts disabled by netconsole.
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