Day 20 - hwmon Subsystem (Hardware Monitoring)

Overview

In this lab, we migrate a custom I2C sensor driver from a manual sysfs interface to the Linux hwmon subsystem.

This introduces: - Standardized sensor interface - Kernel-managed sysfs structure - Cleaner separation between standard data and debug controls

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Why hwmon?

Previously (Day19), we implemented:

/sys/bus/i2c/devices/1-0048/temp_input

This approach has limitations:

• Not standardized
• Not compatible with tools like sensors
• Hard to scale for multiple sensor types

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hwmon Design

hwmon provides a standard interface for sensors:

/sys/class/hwmon/hwmonX/
    ├── name
    ├── temp1_input
    ├── temp1_max
    ├── in1_input
    └── ...

Key Concepts

Concept	Description
hwmon device	Logical sensor device
channel	Sensor type (temp, voltage, fan...)
attribute	sysfs entry (temp1_input)
hwmon_ops	callback interface
hwmon_chip_info	driver descriptor

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Driver Architecture

1. Channel Definition

HWMON_CHANNEL_INFO(temp, HWMON_T_INPUT)

Defines: - one temperature sensor - exposes temp1_input

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2. Visibility Control

.is_visible = myi2c_sensor_hwmon_is_visible

Controls: - which attributes exist - permissions (0444 = read-only)

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3. Read Callback

.read = myi2c_sensor_hwmon_read

Maps:

hwmon attr	driver function
temp1_input	myi2c_sensor_read_temp_mc

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4. hwmon Registration

devm_hwmon_device_register_with_info(...)

Creates: - /sys/class/hwmon/hwmonX/ - standard attributes automatically

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Data Flow

userspace
   ↓
temp1_input
   ↓
hwmon_read()
   ↓
myi2c_sensor_read_temp_mc()
   ↓
I2C register access

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Unit Convention

hwmon requires:

• temperature = milli-degree Celsius
• integer only (no float)

Example:

Raw	Celsius	hwmon
2650	26.50°C	26500

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Custom Attributes (extra_groups)

Non-standard attributes:

• chip_id
• temp_raw
• config
• status
• update_period
• update_once

Are attached via:

extra_groups

Result:

/sys/class/hwmon/hwmonX/
    ├── temp1_input   (standard)
    ├── temp_raw      (custom)
    ├── config
    └── ...

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Migration Summary

Feature	Day19	Day20
temp interface	temp_input	temp1_input
sysfs location	i2c device	hwmon
standardization	❌	✅
extensibility	low	high

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Key Design Insight

Do not reinvent interfaces — use subsystem standards

• Standard → hwmon
• Custom → extra_groups

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When to use hwmon

Use hwmon when your device provides:

• temperature
• voltage
• current
• fan speed
• power

Do NOT use hwmon for:

• arbitrary device control
• non-sensor devices

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Takeaway

• hwmon separates data plane (sensor values) from control/debug plane
• kernel provides structure → driver provides data
• consistent interface improves ecosystem compatibility