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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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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: dev->xmit_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 xmit_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 -1 when the spin lock fails.
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The locking there should also properly protect against
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set_multicast_list
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Context: BHs disabled
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Notes: netif_queue_stopped() is guaranteed false
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Interrupts must be enabled when calling hard_start_xmit.
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(Interrupts must also be enabled when enabling the BH handler.)
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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: dev->xmit_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: dev->xmit_lock spinlock.
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Context: BHs disabled
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dev->poll:
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Synchronization: __LINK_STATE_RX_SCHED bit in dev->state. See
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dev_close code and comments in net/core/dev.c for more info.
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Context: softirq
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