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libsensehst-cpp CHANGELOG

October 2025 -- GPIO

The GPIO library API was changed alongside the migration of the Raspberry Pi OS from Bookworm to Trixie.

This project code was initially developed using the libgpiod v1.x API. With the v2.2.x version now packaged in Debian Trixie, the GPIO functions have been rewritten.

New GPIO API design

According to the libgpiod V2: New Major Release with a Ton of New Features, Libgpiod v2.x has been redesigned to improve usability and provide a flexible API and enhanced platform support. The new version features a streamlined API that is less prone to errors, supports GObject and D-Bus integration, and provides a modern model for GPIO programming. Its internal structures have been reorganised to include immutable snapshots and expanded language bindings.

The public interface now enables line requests to be grouped and line parameters to be configured more precisely, including bias, drive and active low settings. Requests can also be made across multiple lines at once.

Changes to the code in sensehat.cpp

Data structures

On line 89 of the sensehat.cpp file is an array that stores the status of the GPIO line request objects.

static struct gpiod_chip *gpio_chip;
#define GPIOLIST 7
const uint8_t gpio_pinlist[GPIOLIST] = {5, 6, 16, 17, 22, 26, 27};
static struct gpiod_line_request *gpio_line[GPIOLIST];

/* Forward declaration of helper (defined later). */
static struct gpiod_line_request *_request_line(unsigned int pin,
                                                gpio_dir_t direction);

Functions

A first helper function, _open_first_gpiochip(), has been added to dynamically detect and open the first available /dev/gpiochipN, rather than using a hard-coded path.This is similar to what was done for the framebuffer device used to drive the LED matrix.

A second helper function, _request_line(pin, direction), has been added that encapsulates the creation of gpiod_line_settings and gpiod_line_config, and calls gpiod_chip_request_lines().

Replacing read/write calls with gpiod_line_request_get_value(request, offset) and gpiod_line_request_set_value(request, offset, value). The new API requires you to specify the row offsets. We then use these to read or write values according to the defined pin number.

All the requests done with gpiod_line_request are systematically released via the gpiod_line_request_release() call in the senseShutdown() function.

Implementation note

The added helpers are basic and are designed for simple reading and writing operations that students will find easy to understand. They can be enhanced if you require more comprehensive features, such as pull-up/down, debouncing and edge detection.

October 2025 -- PWM

Simplified PWM Management with Persistent File Descriptors

The PWM implementation has been upgraded to use persistent file descriptors rather than opening and closing sysfs files on each operation. This approach provides significant operational and administrative benefits compared to the manual setup documented in docs/pwm.md.

Old Approach Overhead

The previous documentation required:

  • Creation and management of a custom systemd service (pwm-export.service)
  • A shell script (pwm-export.sh) to export PWM channels at boot time

New Approach Benefits

The library now handles PWM channel management transparently:

  • Automatic initialization: PWM channels are initialized during senseInit()
  • Persistent file descriptors: Files are opened once and kept open (stored in pwm_fd_period[2], pwm_fd_duty[2], and pwm_fd_enable[2])
  • Efficient operations: Subsequent PWM calls avoid filesystem overhead by reusing open descriptors
  • Clean shutdown: All file descriptors are properly closed in senseShutdown()

Implementation Details

Three pairs of persistent file descriptors are maintained per PWM channel:

/* Persistent file descriptors for pwm sysfs files per channel.
 * Index 0 => pwm0, index 1 => pwm1. -1 means not opened. */
static int pwm_fd_period[2] = {-1, -1};
static int pwm_fd_duty[2] = {-1, -1};
static int pwm_fd_enable[2] = {-1, -1};

This design minimizes system calls and I/O overhead, making PWM control more responsive.