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17.3. The net_device Structure in Detail

The net_device structure is at the very core of the network driver layer and deserves a complete description. This list describes all the fields, but more to provide a reference than to be memorized. The rest of this chapter briefly describes each field as soon as it is used in the sample code, so you don't need to keep referring back to this section.

17.3.1. Global Information

The first part of struct net_device is composed of the following fields:

char name[IFNAMSIZ];

The name of the device. If the name set by the driver contains a %d format string, register_netdev replaces it with a number to make a unique name; assigned numbers start at 0.

unsigned long state;

Device state. The field includes several flags. Drivers do not normally manipulate these flags directly; instead, a set of utility functions has been provided. These functions are discussed shortly when we get into driver operations.

struct net_device *next;

Pointer to the next device in the global linked list. This field shouldn't be touched by the driver.

int (*init)(struct net_device *dev);

An initialization function. If this pointer is set, the function is called by register_netdev to complete the initialization of the net_device structure. Most modern network drivers do not use this function any longer; instead, initialization is performed before registering the interface.

17.3.2. Hardware Information

The following fields contain low-level hardware information for relatively simple devices. They are a holdover from the earlier days of Linux networking; most modern drivers do make use of them (with the possible exception of if_port). We list them here for completeness.

unsigned long rmem_end;

unsigned long rmem_start;

unsigned long mem_end;

unsigned long mem_start;

Device memory information. These fields hold the beginning and ending addresses of the shared memory used by the device. If the device has different receive and transmit memories, the mem fields are used for transmit memory and the rmem fields for receive memory. The rmem fields are never referenced outside of the driver itself. By convention, the end fields are set so that end - start is the amount of available onboard memory.

unsigned long base_addr;

The I/O base address of the network interface. This field, like the previous ones, is assigned by the driver during the device probe. The ifconfig command can be used to display or modify the current value. The base_addr can be explicitly assigned on the kernel command line at system boot (via the neTDev= parameter) or at module load time. The field, like the memory fields described above, is not used by the kernel.

unsigned char irq;

The assigned interrupt number. The value of dev->irq is printed by ifconfig when interfaces are listed. This value can usually be set at boot or load time and modified later using ifconfig.

unsigned char if_port;

The port in use on multiport devices. This field is used, for example, with devices that support both coaxial (IF_PORT_10BASE2) and twisted-pair (IF_PORT_100BASET) Ethernet connections. The full set of known port types is defined in <linux/netdevice.h>.

unsigned char dma;

The DMA channel allocated by the device. The field makes sense only with some peripheral buses, such as ISA. It is not used outside of the device driver itself but for informational purposes (in ifconfig).

17.3.3. Interface Information

Most of the information about the interface is correctly set up by the ether_setup function (or whatever other setup function is appropriate for the given hardware type). Ethernet cards can rely on this general-purpose function for most of these fields, but the flags and dev_addr fields are device specific and must be explicitly assigned at initialization time.

Some non-Ethernet interfaces can use helper functions similar to ether_setup. drivers/net/net_init.c exports a number of such functions, including the following:

void ltalk_setup(struct net_device *dev);

Sets up the fields for a LocalTalk device

void fc_setup(struct net_device *dev);

Initializes fields for fiber-channel devices

void fddi_setup(struct net_device *dev);

Configures an interface for a Fiber Distributed Data Interface (FDDI) network

void hippi_setup(struct net_device *dev);

Prepares fields for a High-Performance Parallel Interface (HIPPI) high-speed interconnect driver

void tr_setup(struct net_device *dev);

Handles setup for token ring network interfaces

Most devices are covered by one of these classes. If yours is something radically new and different, however, you need to assign the following fields by hand:

unsigned short hard_header_len;

The hardware header length, that is, the number of octets that lead the transmitted packet before the IP header, or other protocol information. The value of hard_header_len is 14 (ETH_HLEN) for Ethernet interfaces.

unsigned mtu;

The maximum transfer unit (MTU). This field is used by the network layer to drive packet transmission. Ethernet has an MTU of 1500 octets (ETH_DATA_LEN). This value can be changed with ifconfig.

unsigned long tx_queue_len;

The maximum number of frames that can be queued on the device's transmission queue. This value is set to 1000 by ether_setup, but you can change it. For example, plip uses 10 to avoid wasting system memory (plip has a lower throughput than a real Ethernet interface).

unsigned short type;

The hardware type of the interface. The type field is used by ARP to determine what kind of hardware address the interface supports. The proper value for Ethernet interfaces is ARPHRD_ETHER, and that is the value set by ether_setup. The recognized types are defined in <linux/if_arp.h>.

unsigned char addr_len;

unsigned char broadcast[MAX_ADDR_LEN];

unsigned char dev_addr[MAX_ADDR_LEN];

Hardware (MAC) address length and device hardware addresses. The Ethernet address length is six octets (we are referring to the hardware ID of the interface board), and the broadcast address is made up of six 0xff octets; ether_setup arranges for these values to be correct. The device address, on the other hand, must be read from the interface board in a device-specific way, and the driver should copy it to dev_addr. The hardware address is used to generate correct Ethernet headers before the packet is handed over to the driver for transmission. The snull device doesn't use a physical interface, and it invents its own hardware address.

unsigned short flags;

int features;

Interface flags (detailed next).

The flags field is a bit mask including the following bit values. The IFF_ prefix stands for "interface flags." Some flags are managed by the kernel, and some are set by the interface at initialization time to assert various capabilities and other features of the interface. The valid flags, which are defined in <linux/if.h>, are:


This flag is read-only for the driver. The kernel turns it on when the interface is active and ready to transfer packets.


This flag (maintained by the networking code) states that the interface allows broadcasting. Ethernet boards do.


This marks debug mode. The flag can be used to control the verbosity of your printk calls or for other debugging purposes. Although no in-tree driver currently uses this flag, it can be set and reset by user programs via ioctl, and your driver can use it. The misc-progs/netifdebug program can be used to turn the flag on and off.


This flag should be set only in the loopback interface. The kernel checks for IFF_LOOPBACK instead of hardwiring the lo name as a special interface.


This flag signals that the interface is connected to a point-to-point link. It is set by the driver or, sometimes, by ifconfig. For example, plip and the PPP driver have it set.


This means that the interface can't perform ARP. For example, point-to-point interfaces don't need to run ARP, which would only impose additional traffic without retrieving useful information. snull runs without ARP capabilities, so it sets the flag.


This flag is set (by the networking code) to activate promiscuous operation. By default, Ethernet interfaces use a hardware filter to ensure that they receive broadcast packets and packets directed to that interface's hardware address only. Packet sniffers such as tcpdump set promiscuous mode on the interface in order to retrieve all packets that travel on the interface's transmission medium.


This flag is set by drivers to mark interfaces that are capable of multicast transmission. ether_setup sets IFF_MULTICAST by default, so if your driver does not support multicast, it must clear the flag at initialization time.


This flag tells the interface to receive all multicast packets. The kernel sets it when the host performs multicast routing, only if IFF_MULTICAST is set. IFF_ALLMULTI is read-only for the driver. Multicast flags are used in Section 17.14 later in this chapter.



These flags are used by the load equalization code. The interface driver doesn't need to know about them.



These flags signal that the device is capable of switching between multiple media types; for example, unshielded twisted pair (UTP) versus coaxial Ethernet cables. If IFF_AUTOMEDIA is set, the device selects the proper medium automatically. In practice, the kernel makes no use of either flag.


This flag, set by the driver, indicates that the address of this interface can change. It is not currently used by the kernel.


This flag indicates that the interface is up and running. It is mostly present for BSD compatibility; the kernel makes little use of it. Most network drivers need not worry about IFF_RUNNING.


This flag is unused in Linux, but it exists for BSD compatibility.

When a program changes IFF_UP, the open or stop device method is called. Furthermore, when IFF_UP or any other flag is modified, the set_multicast_list method is invoked. If the driver needs to perform some action in response to a modification of the flags, it must take that action in set_multicast_list. For example, when IFF_PROMISC is set or reset, set_multicast_list must notify the onboard hardware filter. The responsibilities of this device method are outlined in Section 17.14.

The features field of the net_device structure is set by the driver to tell the kernel about any special hardware capabilities that this interface has. We will discuss some of these features; others are beyond the scope of this book. The full set is:



Both of these flags control the use of scatter/gather I/O. If your interface can transmit a packet that has been split into several distinct memory segments, you should set NETIF_F_SG. Of course, you have to actually implement the scatter/gather I/O (we describe how that is done in the Section 17.5.3). NETIF_F_FRAGLIST states that your interface can cope with packets that have been fragmented; only the loopback driver does this in 2.6.

Note that the kernel does not perform scatter/gather I/O to your device if it does not also provide some form of checksumming as well. The reason is that, if the kernel has to make a pass over a fragmented ("nonlinear") packet to calculate the checksum, it might as well copy the data and coalesce the packet at the same time.




These flags are all ways of telling the kernel that it need not apply checksums to some or all packets leaving the system by this interface. Set NETIF_F_IP_CSUM if your interface can checksum IP packets but not others. If no checksums are ever required for this interface, set NETIF_F_NO_CSUM. The loopback driver sets this flag, and snull does, too; since packets are only transferred through system memory, there is (one hopes!) no opportunity for them to be corrupted, and no need to check them. If your hardware does checksumming itself, set NETIF_F_HW_CSUM.


Set this flag if your device can perform DMA to high memory. In the absence of this flag, all packet buffers provided to your driver are allocated in low memory.





These options describe your hardware's support for 802.1q VLAN packets. VLAN support is beyond what we can cover in this chapter. If VLAN packets confuse your device (which they really shouldn't), set the NETIF_F_VLAN_CHALLENGED flag.


Set this flag if your device can perform TCP segmentation offloading. TSO is an advanced feature that we cannot cover here.

17.3.4. The Device Methods

As happens with the char and block drivers, each network device declares the functions that act on it. Operations that can be performed on network interfaces are listed in this section. Some of the operations can be left NULL, and others are usually untouched because ether_setup assigns suitable methods to them.

Device methods for a network interface can be divided into two groups: fundamental and optional. Fundamental methods include those that are needed to be able to use the interface; optional methods implement more advanced functionalities that are not strictly required. The following are the fundamental methods:

int (*open)(struct net_device *dev);

Opens the interface. The interface is opened whenever ifconfig activates it. The open method should register any system resource it needs (I/O ports, IRQ, DMA, etc.), turn on the hardware, and perform any other setup your device requires.

int (*stop)(struct net_device *dev);

Stops the interface. The interface is stopped when it is brought down. This function should reverse operations performed at open time.

int (*hard_start_xmit) (struct sk_buff *skb, struct net_device *dev);

Method that initiates the transmission of a packet. The full packet (protocol headers and all) is contained in a socket buffer (sk_buff) structure. Socket buffers are introduced later in this chapter.

int (*hard_header) (struct sk_buff *skb, struct net_device *dev, unsigned

short type, void *daddr, void *saddr, unsigned len);

Function (called before hard_start_xmit) that builds the hardware header from the source and destination hardware addresses that were previously retrieved; its job is to organize the information passed to it as arguments into an appropriate, device-specific hardware header. eth_header is the default function for Ethernet-like interfaces, and ether_setup assigns this field accordingly.

int (*rebuild_header)(struct sk_buff *skb);

Function used to rebuild the hardware header after ARP resolution completes but before a packet is transmitted. The default function used by Ethernet devices uses the ARP support code to fill the packet with missing information.

void (*tx_timeout)(struct net_device *dev);

Method called by the networking code when a packet transmission fails to complete within a reasonable period, on the assumption that an interrupt has been missed or the interface has locked up. It should handle the problem and resume packet transmission.

struct net_device_stats *(*get_stats)(struct net_device *dev);

Whenever an application needs to get statistics for the interface, this method is called. This happens, for example, when ifconfig or netstat -i is run. A sample implementation for snull is introduced in Section 17.13.

int (*set_config)(struct net_device *dev, struct ifmap *map);

Changes the interface configuration. This method is the entry point for configuring the driver. The I/O address for the device and its interrupt number can be changed at runtime using set_config. This capability can be used by the system administrator if the interface cannot be probed for. Drivers for modern hardware normally do not need to implement this method.

The remaining device operations are optional:

int weight;

int (*poll)(struct net_device *dev; int *quota);

Method provided by NAPI-compliant drivers to operate the interface in a polled mode, with interrupts disabled. NAPI (and the weight field) are covered in Section 17.8.

void (*poll_controller)(struct net_device *dev);

Function that asks the driver to check for events on the interface in situations where interrupts are disabled. It is used for specific in-kernel networking tasks, such as remote consoles and kernel debugging over the network.

int (*do_ioctl)(struct net_device *dev, struct ifreq *ifr, int cmd);

Performs interface-specific ioctl commands. (Implementation of those commands is described in Section 17.12.) The corresponding field in struct net_device can be left as NULL if the interface doesn't need any interface-specific commands.

void (*set_multicast_list)(struct net_device *dev);

Method called when the multicast list for the device changes and when the flags change. See the Section 17.14 for further details and a sample implementation.

int (*set_mac_address)(struct net_device *dev, void *addr);

Function that can be implemented if the interface supports the ability to change its hardware address. Many interfaces don't support this ability at all. Others use the default eth_mac_addr implementation (from drivers/net/net_init.c). eth_mac_addr only copies the new address into dev->dev_addr, and it does so only if the interface is not running. Drivers that use eth_mac_addr should set the hardware MAC address from dev->dev_addr in their open method.

int (*change_mtu)(struct net_device *dev, int new_mtu);

Function that takes action if there is a change in the maximum transfer unit (MTU) for the interface. If the driver needs to do anything particular when the MTU is changed by the user, it should declare its own function; otherwise, the default does the right thing. snull has a template for the function if you are interested.

int (*header_cache) (struct neighbour *neigh, struct hh_cache *hh);

header_cache is called to fill in the hh_cache structure with the results of an ARP query. Almost all Ethernet-like drivers can use the default eth_header_cache implementation.

int (*header_cache_update) (struct hh_cache *hh, struct net_device *dev,

unsigned char *haddr);

Method that updates the destination address in the hh_cache structure in response to a change. Ethernet devices use eth_header_cache_update.

int (*hard_header_parse) (struct sk_buff *skb, unsigned char *haddr);

The hard_header_parse method extracts the source address from the packet contained in skb, copying it into the buffer at haddr. The return value from the function is the length of that address. Ethernet devices normally use eth_header_parse.

17.3.5. Utility Fields

The remaining struct net_device data fields are used by the interface to hold useful status information. Some of the fields are used by ifconfig and netstat to provide the user with information about the current configuration. Therefore, an interface should assign values to these fields:

unsigned long trans_start;

unsigned long last_rx;

Fields that hold a jiffies value. The driver is responsible for updating these values when transmission begins and when a packet is received, respectively. The trans_start value is used by the networking subsystem to detect transmitter lockups. last_rx is currently unused, but the driver should maintain this field anyway to be prepared for future use.

int watchdog_timeo;

The minimum time (in jiffies) that should pass before the networking layer decides that a transmission timeout has occurred and calls the driver's tx_timeout function.

void *priv;

The equivalent of filp->private_data. In modern drivers, this field is set by alloc_netdev and should not be accessed directly; use netdev_priv instead.

struct dev_mc_list *mc_list;

int mc_count;

Fields that handle multicast transmission. mc_count is the count of items in mc_list. See the Section 17.14 for further details.

spinlock_t xmit_lock;

int xmit_lock_owner;

The xmit_lock is used to avoid multiple simultaneous calls to the driver's hard_start_xmit function. xmit_lock_owner is the number of the CPU that has obtained xmit_lock. The driver should make no changes to these fields.

There are other fields in struct net_device, but they are not used by network drivers.

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