Demystifying Kconfig: Understanding Network Device Support Configuration

Navigating the Linux kernel configuration can be daunting, especially with the myriad options available. Network device support is a crucial component for any system that relies on network connectivity. Today, we'll break down a segment of the Kconfig file that handles this, helping you understand how to configure network devices in the Linux kernel.

Network Device Support in Kconfig

In the Linux kernel, network device support is defined using the NETDEVICES configuration option. This option and its dependencies ensure that your system can support various network interfaces and protocols. Let’s take a closer look at how this is structured in a Kconfig file.

Basic Structure

Here’s the Kconfig entry for NETDEVICES, which is the primary switch for network device support

config NETDEVICES default y if UML depends on NET bool "Network device support" help You can say N here if you don't intend to connect your Linux box to any other computer at all. You'll have to say Y if your computer contains a network card that you want to use under Linux. If you are going to run SLIP or PPP over telephone line or null modem cable you need say Y here. Connecting two machines with parallel ports using PLIP needs this, as well as AX.25/KISS for sending Internet traffic over amateur radio links. If unsure, say Y.

• config NETDEVICES: This keyword introduces the configuration option for network device support.
• default y if UML: This line means the default value is yes if you are using User Mode Linux (UML). Otherwise, it follows the general rule set by the user.
• depends on NET: This option is dependent on the general network support being enabled (NET).
• bool "Network device support": This specifies that the option is a boolean (yes/no) and provides a description seen in the menuconfig interface.
• help: Provides additional information about this option, guiding users on when they should enable it.

Conditional Configuration: The if Block

In Kconfig, you can group related options under a condition. For NETDEVICES, we use an if block to indicate that the following options are only relevant if NETDEVICES is enabled.

#All the following symbols are dependent on NETDEVICES - do not repeat # that for each of the symbols. if NETDEVICES config MII tristate config NET_CORE default y bool "Network core driver support" help You can say N here if you do not intend to use any of the networking core drivers (i.e. VLAN, bridging, bonding, etc.)

Detailed Configurations

Within the if NETDEVICES block, we define additional options like MII and NET_CORE.

MII Configuration

config MII tristate

• tristate: This specifies that the MII option can be built into the kernel (Y), built as a module (M), or not included at all (N). The MII option typically refers to the Media Independent Interface, crucial for Ethernet devices.

Network Core Driver Support

config NET_CORE default y bool "Network core driver support" help You can say N here if you do not intend to use any of the networking core drivers (i.e. VLAN, bridging, bonding, etc.)

• default y: This means the option is enabled by default.
• bool "Network core driver support": This boolean option controls the inclusion of core network drivers, essential for advanced networking features like VLAN and bonding.

Advanced Options: Bonding Driver Support

Nested within NET_CORE is another configuration option for the bonding driver:

if NET_CORE config BONDING tristate "Bonding driver support" depends on INET depends on IPV6 || IPV6=n depends on TLS || TLS_DEVICE=n help Say 'Y' or 'M' if you wish to be able to 'bond' multiple Ethernet Channels together. This is called 'Etherchannel' by Cisco, 'Trunking' by Sun, 802.3ad by the IEEE, and 'Bonding' in Linux. The driver supports multiple bonding modes to allow for both high performance and high availability operation.

• config BONDING: This entry defines support for Ethernet bonding, allowing multiple network interfaces to be combined for improved performance or redundancy.
• tristate "Bonding driver support": This allows the bonding driver to be compiled directly into the kernel, as a module, or not at all.
• depends on INET: The bonding driver depends on basic internet protocol (INET) support.
• depends on IPV6 || IPV6=n: The bonding driver can only be enabled if IPv6 is either enabled or not selected.
• depends on TLS || TLS_DEVICE=n: Similar dependency logic applies to Transport Layer Security (TLS).

By demystifying Kconfig files, you gain the power to customize your Linux kernel precisely to your needs, ensuring it is lean, efficient, and capable of supporting the desired features.