300 lines
9.3 KiB
ReStructuredText
300 lines
9.3 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
=====================================
|
|
Network Devices, the Kernel, and You!
|
|
=====================================
|
|
|
|
|
|
Introduction
|
|
============
|
|
The following is a random collection of documentation regarding
|
|
network devices.
|
|
|
|
struct net_device lifetime rules
|
|
================================
|
|
Network device structures need to persist even after module is unloaded and
|
|
must be allocated with alloc_netdev_mqs() and friends.
|
|
If device has registered successfully, it will be freed on last use
|
|
by free_netdev(). This is required to handle the pathological case cleanly
|
|
(example: ``rmmod mydriver </sys/class/net/myeth/mtu``)
|
|
|
|
alloc_netdev_mqs() / alloc_netdev() reserve extra space for driver
|
|
private data which gets freed when the network device is freed. If
|
|
separately allocated data is attached to the network device
|
|
(netdev_priv()) then it is up to the module exit handler to free that.
|
|
|
|
There are two groups of APIs for registering struct net_device.
|
|
First group can be used in normal contexts where ``rtnl_lock`` is not already
|
|
held: register_netdev(), unregister_netdev().
|
|
Second group can be used when ``rtnl_lock`` is already held:
|
|
register_netdevice(), unregister_netdevice(), free_netdevice().
|
|
|
|
Simple drivers
|
|
--------------
|
|
|
|
Most drivers (especially device drivers) handle lifetime of struct net_device
|
|
in context where ``rtnl_lock`` is not held (e.g. driver probe and remove paths).
|
|
|
|
In that case the struct net_device registration is done using
|
|
the register_netdev(), and unregister_netdev() functions:
|
|
|
|
.. code-block:: c
|
|
|
|
int probe()
|
|
{
|
|
struct my_device_priv *priv;
|
|
int err;
|
|
|
|
dev = alloc_netdev_mqs(...);
|
|
if (!dev)
|
|
return -ENOMEM;
|
|
priv = netdev_priv(dev);
|
|
|
|
/* ... do all device setup before calling register_netdev() ...
|
|
*/
|
|
|
|
err = register_netdev(dev);
|
|
if (err)
|
|
goto err_undo;
|
|
|
|
/* net_device is visible to the user! */
|
|
|
|
err_undo:
|
|
/* ... undo the device setup ... */
|
|
free_netdev(dev);
|
|
return err;
|
|
}
|
|
|
|
void remove()
|
|
{
|
|
unregister_netdev(dev);
|
|
free_netdev(dev);
|
|
}
|
|
|
|
Note that after calling register_netdev() the device is visible in the system.
|
|
Users can open it and start sending / receiving traffic immediately,
|
|
or run any other callback, so all initialization must be done prior to
|
|
registration.
|
|
|
|
unregister_netdev() closes the device and waits for all users to be done
|
|
with it. The memory of struct net_device itself may still be referenced
|
|
by sysfs but all operations on that device will fail.
|
|
|
|
free_netdev() can be called after unregister_netdev() returns on when
|
|
register_netdev() failed.
|
|
|
|
Device management under RTNL
|
|
----------------------------
|
|
|
|
Registering struct net_device while in context which already holds
|
|
the ``rtnl_lock`` requires extra care. In those scenarios most drivers
|
|
will want to make use of struct net_device's ``needs_free_netdev``
|
|
and ``priv_destructor`` members for freeing of state.
|
|
|
|
Example flow of netdev handling under ``rtnl_lock``:
|
|
|
|
.. code-block:: c
|
|
|
|
static void my_setup(struct net_device *dev)
|
|
{
|
|
dev->needs_free_netdev = true;
|
|
}
|
|
|
|
static void my_destructor(struct net_device *dev)
|
|
{
|
|
some_obj_destroy(priv->obj);
|
|
some_uninit(priv);
|
|
}
|
|
|
|
int create_link()
|
|
{
|
|
struct my_device_priv *priv;
|
|
int err;
|
|
|
|
ASSERT_RTNL();
|
|
|
|
dev = alloc_netdev(sizeof(*priv), "net%d", NET_NAME_UNKNOWN, my_setup);
|
|
if (!dev)
|
|
return -ENOMEM;
|
|
priv = netdev_priv(dev);
|
|
|
|
/* Implicit constructor */
|
|
err = some_init(priv);
|
|
if (err)
|
|
goto err_free_dev;
|
|
|
|
priv->obj = some_obj_create();
|
|
if (!priv->obj) {
|
|
err = -ENOMEM;
|
|
goto err_some_uninit;
|
|
}
|
|
/* End of constructor, set the destructor: */
|
|
dev->priv_destructor = my_destructor;
|
|
|
|
err = register_netdevice(dev);
|
|
if (err)
|
|
/* register_netdevice() calls destructor on failure */
|
|
goto err_free_dev;
|
|
|
|
/* If anything fails now unregister_netdevice() (or unregister_netdev())
|
|
* will take care of calling my_destructor and free_netdev().
|
|
*/
|
|
|
|
return 0;
|
|
|
|
err_some_uninit:
|
|
some_uninit(priv);
|
|
err_free_dev:
|
|
free_netdev(dev);
|
|
return err;
|
|
}
|
|
|
|
If struct net_device.priv_destructor is set it will be called by the core
|
|
some time after unregister_netdevice(), it will also be called if
|
|
register_netdevice() fails. The callback may be invoked with or without
|
|
``rtnl_lock`` held.
|
|
|
|
There is no explicit constructor callback, driver "constructs" the private
|
|
netdev state after allocating it and before registration.
|
|
|
|
Setting struct net_device.needs_free_netdev makes core call free_netdevice()
|
|
automatically after unregister_netdevice() when all references to the device
|
|
are gone. It only takes effect after a successful call to register_netdevice()
|
|
so if register_netdevice() fails driver is responsible for calling
|
|
free_netdev().
|
|
|
|
free_netdev() is safe to call on error paths right after unregister_netdevice()
|
|
or when register_netdevice() fails. Parts of netdev (de)registration process
|
|
happen after ``rtnl_lock`` is released, therefore in those cases free_netdev()
|
|
will defer some of the processing until ``rtnl_lock`` is released.
|
|
|
|
Devices spawned from struct rtnl_link_ops should never free the
|
|
struct net_device directly.
|
|
|
|
.ndo_init and .ndo_uninit
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
``.ndo_init`` and ``.ndo_uninit`` callbacks are called during net_device
|
|
registration and de-registration, under ``rtnl_lock``. Drivers can use
|
|
those e.g. when parts of their init process need to run under ``rtnl_lock``.
|
|
|
|
``.ndo_init`` runs before device is visible in the system, ``.ndo_uninit``
|
|
runs during de-registering after device is closed but other subsystems
|
|
may still have outstanding references to the netdevice.
|
|
|
|
MTU
|
|
===
|
|
Each network device has a Maximum Transfer Unit. The MTU does not
|
|
include any link layer protocol overhead. Upper layer protocols must
|
|
not pass a socket buffer (skb) to a device to transmit with more data
|
|
than the mtu. The MTU does not include link layer header overhead, so
|
|
for example on Ethernet if the standard MTU is 1500 bytes used, the
|
|
actual skb will contain up to 1514 bytes because of the Ethernet
|
|
header. Devices should allow for the 4 byte VLAN header as well.
|
|
|
|
Segmentation Offload (GSO, TSO) is an exception to this rule. The
|
|
upper layer protocol may pass a large socket buffer to the device
|
|
transmit routine, and the device will break that up into separate
|
|
packets based on the current MTU.
|
|
|
|
MTU is symmetrical and applies both to receive and transmit. A device
|
|
must be able to receive at least the maximum size packet allowed by
|
|
the MTU. A network device may use the MTU as mechanism to size receive
|
|
buffers, but the device should allow packets with VLAN header. With
|
|
standard Ethernet mtu of 1500 bytes, the device should allow up to
|
|
1518 byte packets (1500 + 14 header + 4 tag). The device may either:
|
|
drop, truncate, or pass up oversize packets, but dropping oversize
|
|
packets is preferred.
|
|
|
|
|
|
struct net_device synchronization rules
|
|
=======================================
|
|
ndo_open:
|
|
Synchronization: rtnl_lock() semaphore.
|
|
Context: process
|
|
|
|
ndo_stop:
|
|
Synchronization: rtnl_lock() semaphore.
|
|
Context: process
|
|
Note: netif_running() is guaranteed false
|
|
|
|
ndo_do_ioctl:
|
|
Synchronization: rtnl_lock() semaphore.
|
|
Context: process
|
|
|
|
This is only called by network subsystems internally,
|
|
not by user space calling ioctl as it was in before
|
|
linux-5.14.
|
|
|
|
ndo_siocbond:
|
|
Synchronization: rtnl_lock() semaphore.
|
|
Context: process
|
|
|
|
Used by the bonding driver for the SIOCBOND family of
|
|
ioctl commands.
|
|
|
|
ndo_siocwandev:
|
|
Synchronization: rtnl_lock() semaphore.
|
|
Context: process
|
|
|
|
Used by the drivers/net/wan framework to handle
|
|
the SIOCWANDEV ioctl with the if_settings structure.
|
|
|
|
ndo_siocdevprivate:
|
|
Synchronization: rtnl_lock() semaphore.
|
|
Context: process
|
|
|
|
This is used to implement SIOCDEVPRIVATE ioctl helpers.
|
|
These should not be added to new drivers, so don't use.
|
|
|
|
ndo_eth_ioctl:
|
|
Synchronization: rtnl_lock() semaphore.
|
|
Context: process
|
|
|
|
ndo_get_stats:
|
|
Synchronization: rtnl_lock() semaphore, or RCU.
|
|
Context: atomic (can't sleep under RCU)
|
|
|
|
ndo_start_xmit:
|
|
Synchronization: __netif_tx_lock spinlock.
|
|
|
|
When the driver sets NETIF_F_LLTX in dev->features this will be
|
|
called without holding netif_tx_lock. In this case the driver
|
|
has to lock by itself when needed.
|
|
The locking there should also properly protect against
|
|
set_rx_mode. WARNING: use of NETIF_F_LLTX is deprecated.
|
|
Don't use it for new drivers.
|
|
|
|
Context: Process with BHs disabled or BH (timer),
|
|
will be called with interrupts disabled by netconsole.
|
|
|
|
Return codes:
|
|
|
|
* NETDEV_TX_OK everything ok.
|
|
* NETDEV_TX_BUSY Cannot transmit packet, try later
|
|
Usually a bug, means queue start/stop flow control is broken in
|
|
the driver. Note: the driver must NOT put the skb in its DMA ring.
|
|
|
|
ndo_tx_timeout:
|
|
Synchronization: netif_tx_lock spinlock; all TX queues frozen.
|
|
Context: BHs disabled
|
|
Notes: netif_queue_stopped() is guaranteed true
|
|
|
|
ndo_set_rx_mode:
|
|
Synchronization: netif_addr_lock spinlock.
|
|
Context: BHs disabled
|
|
|
|
struct napi_struct synchronization rules
|
|
========================================
|
|
napi->poll:
|
|
Synchronization:
|
|
NAPI_STATE_SCHED bit in napi->state. Device
|
|
driver's ndo_stop method will invoke napi_disable() on
|
|
all NAPI instances which will do a sleeping poll on the
|
|
NAPI_STATE_SCHED napi->state bit, waiting for all pending
|
|
NAPI activity to cease.
|
|
|
|
Context:
|
|
softirq
|
|
will be called with interrupts disabled by netconsole.
|