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1c8c7d6416
Document the expectations about device MTU handling. The documentation about oversize packet handling is probably too loose. IMHO devices should drop oversize packets for robustness, but many devices allow it now. For example, if you set mtu to 1200 bytes, most ether devices will allow a 1500 byte frame in. Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org> Signed-off-by: David S. Miller <davem@davemloft.net>
104 lines
3.8 KiB
Plaintext
104 lines
3.8 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.
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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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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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will be called with interrupts disabled by netconsole.
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