300 lines
9.4 KiB
ReStructuredText
300 lines
9.4 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=====================================
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Network Devices, the Kernel, and You!
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=====================================
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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 lifetime 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 pathological 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()) then it is up to the module exit handler to free that.
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There are two groups of APIs for registering struct net_device.
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First group can be used in normal contexts where ``rtnl_lock`` is not already
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held: register_netdev(), unregister_netdev().
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Second group can be used when ``rtnl_lock`` is already held:
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register_netdevice(), unregister_netdevice(), free_netdevice().
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Simple drivers
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--------------
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Most drivers (especially device drivers) handle lifetime of struct net_device
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in context where ``rtnl_lock`` is not held (e.g. driver probe and remove paths).
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In that case the struct net_device registration is done using
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the register_netdev(), and unregister_netdev() functions:
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.. code-block:: c
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int probe()
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{
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struct my_device_priv *priv;
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int err;
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dev = alloc_netdev_mqs(...);
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if (!dev)
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return -ENOMEM;
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priv = netdev_priv(dev);
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/* ... do all device setup before calling register_netdev() ...
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*/
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err = register_netdev(dev);
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if (err)
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goto err_undo;
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/* net_device is visible to the user! */
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err_undo:
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/* ... undo the device setup ... */
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free_netdev(dev);
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return err;
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}
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void remove()
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{
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unregister_netdev(dev);
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free_netdev(dev);
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}
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Note that after calling register_netdev() the device is visible in the system.
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Users can open it and start sending / receiving traffic immediately,
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or run any other callback, so all initialization must be done prior to
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registration.
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unregister_netdev() closes the device and waits for all users to be done
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with it. The memory of struct net_device itself may still be referenced
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by sysfs but all operations on that device will fail.
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free_netdev() can be called after unregister_netdev() returns on when
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register_netdev() failed.
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Device management under RTNL
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----------------------------
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Registering struct net_device while in context which already holds
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the ``rtnl_lock`` requires extra care. In those scenarios most drivers
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will want to make use of struct net_device's ``needs_free_netdev``
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and ``priv_destructor`` members for freeing of state.
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Example flow of netdev handling under ``rtnl_lock``:
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.. code-block:: c
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static void my_setup(struct net_device *dev)
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{
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dev->needs_free_netdev = true;
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}
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static void my_destructor(struct net_device *dev)
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{
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some_obj_destroy(priv->obj);
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some_uninit(priv);
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}
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int create_link()
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{
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struct my_device_priv *priv;
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int err;
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ASSERT_RTNL();
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dev = alloc_netdev(sizeof(*priv), "net%d", NET_NAME_UNKNOWN, my_setup);
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if (!dev)
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return -ENOMEM;
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priv = netdev_priv(dev);
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/* Implicit constructor */
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err = some_init(priv);
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if (err)
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goto err_free_dev;
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priv->obj = some_obj_create();
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if (!priv->obj) {
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err = -ENOMEM;
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goto err_some_uninit;
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}
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/* End of constructor, set the destructor: */
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dev->priv_destructor = my_destructor;
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err = register_netdevice(dev);
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if (err)
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/* register_netdevice() calls destructor on failure */
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goto err_free_dev;
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/* If anything fails now unregister_netdevice() (or unregister_netdev())
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* will take care of calling my_destructor and free_netdev().
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*/
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return 0;
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err_some_uninit:
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some_uninit(priv);
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err_free_dev:
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free_netdev(dev);
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return err;
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}
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If struct net_device.priv_destructor is set it will be called by the core
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some time after unregister_netdevice(), it will also be called if
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register_netdevice() fails. The callback may be invoked with or without
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``rtnl_lock`` held.
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There is no explicit constructor callback, driver "constructs" the private
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netdev state after allocating it and before registration.
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Setting struct net_device.needs_free_netdev makes core call free_netdevice()
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automatically after unregister_netdevice() when all references to the device
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are gone. It only takes effect after a successful call to register_netdevice()
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so if register_netdevice() fails driver is responsible for calling
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free_netdev().
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free_netdev() is safe to call on error paths right after unregister_netdevice()
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or when register_netdevice() fails. Parts of netdev (de)registration process
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happen after ``rtnl_lock`` is released, therefore in those cases free_netdev()
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will defer some of the processing until ``rtnl_lock`` is released.
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Devices spawned from struct rtnl_link_ops should never free the
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struct net_device directly.
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.ndo_init and .ndo_uninit
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~~~~~~~~~~~~~~~~~~~~~~~~~
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``.ndo_init`` and ``.ndo_uninit`` callbacks are called during net_device
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registration and de-registration, under ``rtnl_lock``. Drivers can use
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those e.g. when parts of their init process need to run under ``rtnl_lock``.
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``.ndo_init`` runs before device is visible in the system, ``.ndo_uninit``
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runs during de-registering after device is closed but other subsystems
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may still have outstanding references to the netdevice.
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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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This is only called by network subsystems internally,
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not by user space calling ioctl as it was in before
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linux-5.14.
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ndo_siocbond:
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Synchronization: rtnl_lock() semaphore.
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Context: process
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Used by the bonding driver for the SIOCBOND family of
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ioctl commands.
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ndo_siocwandev:
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Synchronization: rtnl_lock() semaphore.
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Context: process
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Used by the drivers/net/wan framework to handle
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the SIOCWANDEV ioctl with the if_settings structure.
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ndo_siocdevprivate:
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Synchronization: rtnl_lock() semaphore.
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Context: process
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This is used to implement SIOCDEVPRIVATE ioctl helpers.
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These should not be added to new drivers, so don't use.
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ndo_eth_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: rtnl_lock() semaphore, dev_base_lock rwlock, or RCU.
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Context: atomic (can't sleep under rwlock or RCU)
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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.
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The locking there should also properly protect against
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set_rx_mode. WARNING: 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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* NETDEV_TX_OK everything ok.
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* 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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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:
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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:
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softirq
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will be called with interrupts disabled by netconsole.
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