Linux PWM HAL For Lgpio: A Deep Dive

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Linux PWM HAL for lgpio: A Deep Dive

Hey guys! Today, we're diving deep into the exciting world of the Linux PWM Hardware Abstraction Layer (HAL) for the lgpio framework. This enhancement brings a more robust and efficient way to handle PWM functionalities on ARM Linux platforms, making it a super cool addition for developers and hobbyists alike. Let's break it down!

Overview

At its core, the PWM HAL is all about adding a hardware PWM abstraction layer to the lgpio framework, leveraging the standard Linux /sys/class/pwm interface (sysfs). This approach offers a lightweight, kernel-based PWM API, eliminating the need for daemon dependencies. This is the most future-proof and "Linux-native" way to expose hardware PWM for ARM Linux platforms. Think of it as giving your Raspberry Pi a more direct and efficient way to control things like LED brightness or motor speeds!

The cool part is that this is an optional enhancement, meaning the platform is already production-ready without it. But, for those who need precise timing control—think LED fading, servo motors, or even intricate motor speed control—this feature is a game-changer. The estimated effort to implement this is around 6-9 hours, with the primary dependency being the completion of Task 1.1 (lgpio framework), which is already done. You can find more details in research/03-implementation-roadmap.md Task 2.5 (lines 713-932).

Success Criteria

So, what does success look like for this project? Here's a checklist:

  • Integration: The PWM HAL seamlessly extends the lgpio framework, integrated directly into builder/frameworks/lgpio.py. This ensures a unified and coherent API for both GPIO and PWM functionalities.
  • Core Library: The core library transparently wraps /sys/class/pwm file operations, making it easier to interact with the PWM hardware.
  • Pin Mapping: Board-specific GPIO pin-to-PWM-channel mapping tables are in place, allowing for easy configuration on different Raspberry Pi models.
  • Auto-Detection: The system auto-detects the PWM chip number based on the Pi model. This is especially important because the Pi 5 uses different PWM chips compared to older models.
  • User-Friendly API: The API accepts GPIO pin numbers (making it user-friendly) and internally maps these to the correct chip/channel.
  • Full API Implementation: The complete API is implemented, including init, write, deinit, set_frequency, set_polarity, get_status, and is_enabled.
  • Example Project: An example project demonstrates PWM LED fading, providing a practical use-case for the new HAL.
  • Comprehensive Documentation: The documentation covers kernel module setup, device tree overlays, and permissions, ensuring users have all the information they need.
  • Setup Automation: A systemd service script automates permissions setup, making deployment easier.
  • Testing: Thoroughly tested on both Pi 4 and Pi 5 to ensure compatibility across different hardware.

Implementation Strategy

Let's talk strategy, guys! Here’s how we plan to bring this PWM HAL to life.

1. Integration Approach

We're extending the lgpio framework to create a coherent GPIO + PWM API. This means:

  • Location: Enhancing builder/frameworks/lgpio.py and the lgpio core library.
  • Benefits: Users get one framework for both GPIO and PWM, reducing friction and complexity.
  • Availability: The PWM HAL becomes available when framework = lgpio is set.

2. Kernel Requirements

To make this work, we need to ensure the correct kernel modules and device tree settings are in place:

  • Driver: Use the pwm-bcm2835 (Pi 1-4) or RP1 PWM (Pi 5) kernel module.

  • Device Tree: Users must enable the appropriate device tree overlay in /boot/config.txt:

    dtoverlay=pwm-2chan  # or dtoverlay=pwm for single channel

  • Auto-Detection: Automatically map the Pi model to the correct pwmchip number (Pi 1-4: pwmchip0, Pi 5: pwmchip2/3).

3. API Design

The API is designed to be straightforward and easy to use:

// Constants
#define PWM_NORMAL    0
#define PWM_INVERSED  1

// Core Functions
int pwm_init(int pin, int freq_hz);
int pwm_write(int pin, float duty_cycle_percent);  // 0.0 to 100.0
int pwm_deinit(int pin);

// Extended Functions
int pwm_set_frequency(int pin, int freq_hz);
int pwm_set_polarity(int pin, int polarity);  // PWM_NORMAL or PWM_INVERSED
int pwm_get_status(int pin, int *freq_hz_out, float *duty_cycle_out);
int pwm_is_enabled(int pin);

4. Pin Mapping Strategy

The API accepts GPIO pin numbers, making it more intuitive. Internally, these are mapped to the corresponding pwmchip and channel:

Pi Model GPIO Pin PWM Chip PWM Channel
Pi 1-4 12 pwmchip0 0
Pi 1-4 13 pwmchip0 1
Pi 1-4 18 pwmchip0 0 (alt)
Pi 1-4 19 pwmchip0 1 (alt)
Pi 5 12 pwmchip2 0 (TBD)
Pi 5 13 pwmchip2 1 (TBD)

5. Permissions Strategy

To handle permissions, we’ll provide a setup script run by a systemd service at boot:

  • Setup Script (scripts/setup-pwm-perms.sh):
    • Exports PWM channels.
    • Sets group ownership (root:gpio).
    • Sets permissions (g+rwX).
  • Systemd Unit (scripts/platformio-pwm.service):
    • One-shot service at boot.
    • Runs setup-pwm-perms.sh.
    • Enables after multi-user.target.

Why This Approach?

So, why go with this HAL approach over traditional methods like MMIO or pigpio? Here's a quick comparison:

Aspect Traditional (MMIO/pigpio) This HAL (/sys/class/pwm)
Privilege Requires root or daemon User-level (with setup)
Kernel Updates Breaks with kernel changes Stable kernel interface
Pi 5 Support ❌ Incompatible (RP1 chip) ✅ Works (kernel abstracts)
Architecture Hardware-specific registers Architecture-agnostic
Maintenance High (track register changes) Low (kernel maintains)
Portability Pi-specific Works on all Linux ARM

This HAL offers better portability, lower maintenance, and improved security by running in user-level with proper setup.

Implementation Breakdown (6-9 hours)

Here's a rough breakdown of the implementation tasks:

Core Library (3-4h)

  • [ ] 1h: Core file operations (export, period, duty_cycle, enable).
  • [ ] 1h: API functions (init, write, deinit, set_frequency, set_polarity, get_status).
  • [ ] 1h: Pi model detection, pwmchip auto-detection, GPIO pin mapping.
  • [ ] 1h: Error handling (permissions, missing DTO, invalid pins, busy channels).

Testing & Validation (2-3h)

  • [ ] 1h: Test on Pi 4 (pwmchip0).
  • [ ] 1h: Test on Pi 5 (pwmchip2/3) - different hardware.
  • [ ] 1h: Edge cases (permissions, busy channels, invalid pins, multi-channel).

Documentation & Examples (1-2h)

  • [ ] 30 min: PWM LED fade example project.
  • [ ] 30 min: Setup script (setup-pwm-perms.sh, systemd service).
  • [ ] 30 min: docs/PWM_SETUP.md (setup guide, API reference, troubleshooting).
  • [ ] 30 min: Optional servo control example, advanced usage docs.

Files to Create/Modify

Here's a list of files that will be created or modified during this project:

Core Library:

  • [ ] framework-lgpio/pwm-hal.c - HAL implementation
  • [ ] framework-lgpio/pwm-hal.h - API header

Framework Integration:

  • [ ] builder/frameworks/lgpio.py - Add PWM HAL to build

Examples:

  • [ ] examples/lgpio-pwm-fade/ - LED fade demonstration
  • [ ] Optional: examples/lgpio-pwm-servo/ - Servo control

Setup Scripts:

  • [ ] scripts/setup-pwm-perms.sh - Permissions setup
  • [ ] scripts/platformio-pwm.service - Systemd unit file

Documentation:

  • [ ] docs/PWM_SETUP.md - Comprehensive setup and API guide

Risks & Mitigation

Like any project, there are potential risks. Here's how we plan to mitigate them:

Risk Mitigation
Pi 5 PWM chip mapping unknown Research RP1 datasheet, test on hardware, document findings
Device tree overlay conflicts Document overlay requirements clearly, test with different DT configs
Permissions setup too complex Provide automated script, clear step-by-step guide
Frequency/duty cycle limits vary by Pi model Document hardware limits per model, add runtime validation
No Pi 5 hardware for testing Seek community testers, validate logic via code review

References

Here are some helpful references for those who want to dive deeper:

So there you have it! The Linux PWM HAL for lgpio is an exciting enhancement that promises to bring more efficient and reliable PWM functionalities to ARM Linux platforms. Whether you're fading LEDs or controlling servo motors, this HAL is designed to make your life easier and your projects more awesome!