109 lines
4.1 KiB
Plaintext
109 lines
4.1 KiB
Plaintext
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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 kmalloc. If device has registered successfully,
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it will be freed on last use by free_netdev. This is required to handle the
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pathologic case cleanly (example: rmmod mydriver </sys/class/net/myeth/mtu )
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There are routines in net_init.c to handle the common cases of
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alloc_etherdev, alloc_netdev. These 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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(dev->priv) 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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dev->open:
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Synchronization: rtnl_lock() semaphore.
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Context: process
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dev->stop:
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Synchronization: rtnl_lock() semaphore.
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Context: process
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Note1: netif_running() is guaranteed false
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Note2: dev->poll() is guaranteed to be stopped
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dev->do_ioctl:
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Synchronization: rtnl_lock() semaphore.
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Context: process
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dev->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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dev->hard_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_multicast_list. Note that the use of NETIF_F_LLTX is deprecated.
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Dont 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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dev->tx_timeout:
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Synchronization: netif_tx_lock spinlock.
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Context: BHs disabled
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Notes: netif_queue_stopped() is guaranteed true
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dev->set_multicast_list:
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Synchronization: netif_tx_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 dev->close 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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