Part 6: Device Model and Driver Framework

The Linux device model provides a unified framework for representing devices, their drivers, and their relationships. Understanding this model is essential for writing maintainable, well-integrated drivers.

The Device Model Hierarchy

flowchart TB
    subgraph Buses["Bus Types"]
        PCI["PCI Bus"]
        USB["USB Bus"]
        Platform["Platform Bus"]
        I2C["I2C Bus"]
    end

    subgraph Devices["Devices"]
        D1["Device 1"]
        D2["Device 2"]
        D3["Device 3"]
    end

    subgraph Drivers["Drivers"]
        Dr1["Driver A"]
        Dr2["Driver B"]
    end

    PCI --> D1
    Platform --> D2
    I2C --> D3

    D1 --> Dr1
    D2 --> Dr1
    D3 --> Dr2

    style Buses fill:#738f99,stroke:#0277bd
    style Devices fill:#8f8a73,stroke:#f9a825
    style Drivers fill:#8f7392,stroke:#6a1b9a

Chapter Contents

Chapter	Topic	Key Concepts
6.1	Device Model	Buses, devices, drivers hierarchy
6.2	Kobjects	kobject, kset, reference counting
6.3	Platform Drivers	platform_device, platform_driver
6.4	Probe and Remove	Driver lifecycle, binding
6.5	Managed Resources	devm_* functions, automatic cleanup
6.6	Device Attributes	sysfs interface, DEVICE_ATTR
6.7	Deferred Probe	-EPROBE_DEFER handling

Key Concepts

The Bus-Device-Driver Triangle

flowchart LR
    Bus["Bus"]
    Device["Device"]
    Driver["Driver"]

    Bus -->|"has"| Device
    Bus -->|"has"| Driver
    Device <-->|"match & bind"| Driver

    style Bus fill:#738f99,stroke:#0277bd
    style Device fill:#8f8a73,stroke:#f9a825
    style Driver fill:#8f7392,stroke:#6a1b9a

Bus: Defines how devices are discovered and how drivers are matched
Device: Represents a piece of hardware
Driver: Contains code to control a device

Driver Registration Flow

sequenceDiagram
    participant Driver
    participant Bus
    participant Device

    Driver->>Bus: Register driver
    Bus->>Bus: For each device
    Bus->>Bus: Try match(device, driver)
    Bus-->>Driver: Match found!
    Bus->>Driver: Call probe()
    Driver->>Device: Initialize device

Why Platform Drivers?

Not all devices sit on discoverable buses (like PCI or USB). For:

System-on-Chip (SoC) peripherals
Memory-mapped devices
Devices described in Device Tree

We use the platform bus - a virtual bus for non-discoverable devices.

Examples

This part includes working examples:

platform-driver: Complete platform driver implementation

Prerequisites

Before starting this part, ensure you understand:

Module lifecycle (Part 2)
Character device basics (Part 3)
Memory allocation and mapping (Part 5)