Part 10: PWM, Watchdog, HWMON, LED Drivers

This part covers several specialized device subsystems commonly used in embedded Linux systems. Each subsystem provides a standardized interface for specific types of hardware.

flowchart TB
    subgraph Subsystems["Linux Device Subsystems"]
        PWM["PWM<br/>Pulse Width Modulation"]
        WDT["Watchdog<br/>System Monitoring"]
        HWMON["HWMON<br/>Hardware Monitoring"]
        LED["LED<br/>LED Class"]
    end

    subgraph Interfaces["User Interfaces"]
        PWMFS["/sys/class/pwm/"]
        WDTDEV["/dev/watchdog"]
        HWMONFS["/sys/class/hwmon/"]
        LEDFS["/sys/class/leds/"]
    end

    subgraph Applications["Applications"]
        FAN["Fan Control"]
        MOTOR["Motor Control"]
        BACKLIGHT["Backlight"]
        SAFETY["Safety Monitor"]
        SENSOR["Sensor Monitoring"]
        STATUS["Status Indicators"]
    end

    PWM --> PWMFS
    WDT --> WDTDEV
    HWMON --> HWMONFS
    LED --> LEDFS

    PWMFS --> FAN
    PWMFS --> MOTOR
    PWMFS --> BACKLIGHT
    WDTDEV --> SAFETY
    HWMONFS --> SENSOR
    LEDFS --> STATUS

    style PWM fill:#738f99,stroke:#0277bd
    style WDT fill:#8f7370,stroke:#c62828
    style HWMON fill:#7a8f73,stroke:#2e7d32
    style LED fill:#8f8a73,stroke:#f9a825

What You’ll Learn

PWM Subsystem

PWM chip architecture and implementation
PWM consumer API
Duty cycle and period control
Polarity configuration

Watchdog Drivers

Watchdog architecture
watchdog_device implementation
Timeout configuration
Pretimeout support

Hardware Monitoring (HWMON)

HWMON subsystem architecture
Temperature, voltage, fan speed monitoring
hwmon_chip_info registration
Sensor attributes

LED Class Drivers

LED class framework
Brightness control
LED triggers
Hardware blink support

Subsystem Overview

flowchart LR
    subgraph PWM_Sub["PWM Subsystem"]
        PWMCHIP["pwm_chip"]
        PWMDEV["pwm_device"]
        PWMOPS["pwm_ops"]
        PWMCHIP --> PWMOPS
        PWMCHIP --> PWMDEV
    end

    subgraph WDT_Sub["Watchdog Subsystem"]
        WDTDEV2["watchdog_device"]
        WDTOPS["watchdog_ops"]
        WDTINFO["watchdog_info"]
        WDTDEV2 --> WDTOPS
        WDTDEV2 --> WDTINFO
    end

    subgraph HWMON_Sub["HWMON Subsystem"]
        HWDEV["hwmon_device"]
        HWCHIP["hwmon_chip_info"]
        HWCHAN["hwmon_channel_info"]
        HWDEV --> HWCHIP
        HWCHIP --> HWCHAN
    end

    subgraph LED_Sub["LED Subsystem"]
        LEDCDEV["led_classdev"]
        LEDOPS["brightness_set"]
        TRIGGER["led_trigger"]
        LEDCDEV --> LEDOPS
        LEDCDEV --> TRIGGER
    end

    style PWM_Sub fill:#738f99,stroke:#0277bd
    style WDT_Sub fill:#8f6d72,stroke:#c62828
    style HWMON_Sub fill:#7a8f73,stroke:#2e7d32
    style LED_Sub fill:#8f8a73,stroke:#f9a825

Key Structures

Subsystem	Main Structure	Registration Function
PWM	`struct pwm_chip`	`pwmchip_add()`
Watchdog	`struct watchdog_device`	`watchdog_register_device()`
HWMON	`struct hwmon_chip_info`	`hwmon_device_register_with_info()`
LED	`struct led_classdev`	`led_classdev_register()`
Input	`struct input_dev`	`input_register_device()`

Common Patterns

All these subsystems share common patterns:

Provider/Consumer Model: Drivers implement providers; other drivers/userspace consume
sysfs Interface: Standard attributes exposed in /sys/class/
Device Tree Integration: Properties for configuration
Managed Resources: devm_* variants for automatic cleanup

In This Part

PWM Subsystem - PWM architecture overview
PWM Provider - Implementing pwm_chip
Watchdog Subsystem - Watchdog architecture
Watchdog Driver - Implementing watchdog_device
HWMON Subsystem - Hardware monitoring overview
HWMON Driver - Implementing HWMON sensors
LED Subsystem - LED class drivers
Input Subsystem - Input device framework

Prerequisites

Before starting this part, you should understand:

Platform drivers and Device Tree (Part 8)
Basic driver probe/remove lifecycle (Part 6)
Managed resources (devm_*) - all examples use device-managed allocation
sysfs attributes (Part 6)