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Day 20 - hwmon Subsystem

Summary

Today I migrated my I2C sensor driver from a custom sysfs interface to the hwmon subsystem.

Key changes: - temp_input → temp1_input - manual sysfs → hwmon framework - added hwmon_ops and chip_info - used extra_groups for debug attributes

What I Learned

1. hwmon is a standardized sensor framework

Instead of defining custom sysfs files, Linux already provides: - naming convention - structure - user-space compatibility

2. hwmon separates responsibilities

  • driver → provides data
  • subsystem → handles interface

3. data format is strict

  • temperature must be integer
  • unit is milli-degree Celsius

4. extra_groups is important

Allows mixing: - standard attributes - custom debug attributes

5. devm API simplifies lifecycle

No need for: - manual cleanup - remove() handling

Issues Encountered

hwmon name warning

Problem: 

hwmon: 'myi2c-sensor' is not a valid name

Fix: - use underscore instead of dash

Different values between interfaces

Cause: - update_once is triggered per read

Final Result

/sys/class/hwmon/hwmonX/
    temp1_input
    temp_raw
    config
    status
    update_period

Reflection

This lab clearly shows the importance of:

  • using kernel subsystems
  • not reinventing interfaces
  • designing clean abstraction layers

Next

Potential improvements: - caching (update interval) - support multiple channels - integrate with userspace tools (lm-sensors)