Introduction to Device Drivers

What is a Device Driver?

A device driver is a software component that enables the Linux kernel to communicate with hardware devices. It abstracts low-level details of the hardware and exposes a standard API to the rest of the kernel or user-space.

Think of drivers as translators between the hardware's language (registers, interrupts)  and the kernel's expectations (function calls, system APIs)

Responsibilities of a Driver

• Hardware initialization and configuration
• Interrupt handling
• Read/write operations
• Power management
• Error handling

Why Do We Need Drivers?

Hardware manufacturers build their own chips, peripherals, and boards. Instead of modifying the kernel itself for every new device, the kernel allows drivers to be written separately. This:

• Keeps the kernel general-purpose
• Enables dynamic loading (using modules)
• Supports a wide range of hardware with a common API

Types of Device Drivers in Linux

1 Character Drivers

• Devices accesses byte-by-byte (e.g., keyboards, serial ports)
• User interacts with them via system calls like read() and write()

2 Block Drivers

• Devices accessed in blocks (e.g., hard drives, SD cards)
• Used with the Virtual File System (VFS) and support caching

3 Network Drivers

• Handle packet-based data (Ethernet, Wi-Fi)
• Interface with the networking stack (net_device, NAPI)

4 USB Drivers

• Detect USB devices and manage enumeration, data transfer
• Use the Linux USB core APIs

5 Platform Drivers

• Match devices listed in the device tree or ACPI tables
• Common in embedded Linux systems (ARM boards)

6 Virtual/Pseudo Drivers

• Software-only drivers (e.g., /dev/null, loopback interface)
• Great for learning and simulation

The Linux Driver Model

Linux uses a bus-centric driver model, where:

• Devices are connected to buses (e.g., PCI, USB, platform)
• Drivers are matched to devices using identifiers

Core Components:

• struct device – Represents a device
• struct driver – Represents a driver
• struct bus_type – Represents a communication bus

Each bus (e.g., PCI, USB, SPI) has its own mechanism to match devices with drivers using ID table.

Matching Mechanism:

When a device is detected:

1. It is registered with a bus.
2. The bus looks for a driver with a matching device ID.
3. The driver's probe() function is called to initialize the hardware.