Part 12: Network Device Drivers

Network drivers connect the kernel’s TCP/IP stack to physical or virtual network hardware.

Why Network Drivers Are Different

Network drivers differ from character/block devices in key ways:

Aspect	Char/Block Device	Network Device
Data flow	User initiates read/write	Packets arrive asynchronously
Addressing	/dev node	Interface name (eth0)
Performance	Usually OK with syscalls	Must handle millions of packets/sec
API	file_operations	net_device_ops

The challenge: high packet rates require interrupt mitigation (NAPI).

Mental Model

flowchart TB
    subgraph UserSpace["User Space"]
        APP["Application<br/>send()/recv()"]
    end

    subgraph Kernel["Kernel Network Stack"]
        SOCK["Socket Layer"]
        TCP["TCP/UDP"]
        IP["IP Layer"]
        NETDEV["net_device"]
    end

    subgraph Driver["Your Driver"]
        OPS["net_device_ops"]
        NAPI["NAPI Poll"]
        TX["TX Ring"]
        RX["RX Ring"]
    end

    subgraph HW["Hardware"]
        NIC["Network Controller"]
    end

    APP --> SOCK
    SOCK --> TCP
    TCP --> IP
    IP --> NETDEV
    NETDEV --> OPS
    OPS --> TX
    NAPI --> RX
    TX --> NIC
    NIC --> RX

    style UserSpace fill:#7a8f73,stroke:#2e7d32
    style Kernel fill:#738f99,stroke:#0277bd
    style Driver fill:#8f8a73,stroke:#f9a825
    style HW fill:#826563,stroke:#c62828

Key insight: TX path is synchronous (kernel calls your ndo_start_xmit), but RX path uses polling (NAPI) to avoid interrupt storms.

Chapters

Chapter	What You’ll Learn
Concepts	net_device, sk_buff, and the TX/RX mental model
Driver Skeleton	Typical driver structure and key callbacks
NAPI	Interrupt mitigation through polling
Ethtool	Configuration and statistics via ethtool
Link Management	Carrier status and PHY integration

Example

This part includes ONE complete working example:

Virtual Network Device - Loopback-style driver demonstrating all core concepts

Prerequisites

Kernel module basics (Part 1-2)
Device model and platform drivers (Part 6)
Managed resources (devm_*) - for automatic cleanup
Interrupt handling concepts (Part 7)
DMA basics helpful but not required (Part 5)