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* @file gpio_test.c
* @author Derek Molloy
* @date 19 April 2015
* @brief A kernel module for controlling a GPIO LED/button pair. The device mounts devices via
* sysfs /sys/class/gpio/gpio115 and gpio49. Therefore, this test LKM circuit assumes that an LED
* is attached to GPIO 49 which is on P9_23 and the button is attached to GPIO 115 on P9_27. There
* is no requirement for a custom overlay, as the pins are in their default mux mode states.
* @see
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/gpio.h> // Required for the GPIO functions
#include <linux/interrupt.h> // Required for the IRQ code
MODULE_AUTHOR("Derek Molloy");
MODULE_DESCRIPTION("A Button/LED test driver for the BBB");
static unsigned int gpioLED = 49; ///< hard coding the LED gpio for this example to P9_23 (GPIO49)
static unsigned int gpioButton = 115; ///< hard coding the button gpio for this example to P9_27 (GPIO115)
static unsigned int irqNumber; ///< Used to share the IRQ number within this file
static unsigned int numberPresses = 0; ///< For information, store the number of button presses
static bool ledOn = 0; ///< Is the LED on or off? Used to invert its state (off by default)
/// Function prototype for the custom IRQ handler function -- see below for the implementation
static irq_handler_t ebbgpio_irq_handler(unsigned int irq, void *dev_id, struct pt_regs *regs);
/** @brief The LKM initialization function
* The static keyword restricts the visibility of the function to within this C file. The __init
* macro means that for a built-in driver (not a LKM) the function is only used at initialization
* time and that it can be discarded and its memory freed up after that point. In this example this
* function sets up the GPIOs and the IRQ
* @return returns 0 if successful
static int __init ebbgpio_init(void){
int result = 0;
printk(KERN_INFO "GPIO_TEST: Initializing the GPIO_TEST LKM\n");
// Is the GPIO a valid GPIO number (e.g., the BBB has 4x32 but not all available)
if (!gpio_is_valid(gpioLED)){
printk(KERN_INFO "GPIO_TEST: invalid LED GPIO\n");
return -ENODEV;
// Going to set up the LED. It is a GPIO in output mode and will be on by default
ledOn = true;
gpio_request(gpioLED, "sysfs"); // gpioLED is hardcoded to 49, request it
gpio_direction_output(gpioLED, ledOn); // Set the gpio to be in output mode and on
// gpio_set_value(gpioLED, ledOn); // Not required as set by line above (here for reference)
gpio_export(gpioLED, false); // Causes gpio49 to appear in /sys/class/gpio
// the bool argument prevents the direction from being changed
gpio_request(gpioButton, "sysfs"); // Set up the gpioButton
gpio_direction_input(gpioButton); // Set the button GPIO to be an input
gpio_set_debounce(gpioButton, 200); // Debounce the button with a delay of 200ms
gpio_export(gpioButton, false); // Causes gpio115 to appear in /sys/class/gpio
// the bool argument prevents the direction from being changed
// Perform a quick test to see that the button is working as expected on LKM load
printk(KERN_INFO "GPIO_TEST: The button state is currently: %d\n", gpio_get_value(gpioButton));
// GPIO numbers and IRQ numbers are not the same! This function performs the mapping for us
irqNumber = gpio_to_irq(gpioButton);
printk(KERN_INFO "GPIO_TEST: The button is mapped to IRQ: %d\n", irqNumber);
// This next call requests an interrupt line
result = request_irq(irqNumber, // The interrupt number requested
(irq_handler_t) ebbgpio_irq_handler, // The pointer to the handler function below
IRQF_TRIGGER_RISING, // Interrupt on rising edge (button press, not release)
"ebb_gpio_handler", // Used in /proc/interrupts to identify the owner
NULL); // The *dev_id for shared interrupt lines, NULL is okay
printk(KERN_INFO "GPIO_TEST: The interrupt request result is: %d\n", result);
return result;
/** @brief The LKM cleanup function
* Similar to the initialization function, it is static. The __exit macro notifies that if this
* code is used for a built-in driver (not a LKM) that this function is not required. Used to release the
* GPIOs and display cleanup messages.
static void __exit ebbgpio_exit(void){
printk(KERN_INFO "GPIO_TEST: The button state is currently: %d\n", gpio_get_value(gpioButton));
printk(KERN_INFO "GPIO_TEST: The button was pressed %d times\n", numberPresses);
gpio_set_value(gpioLED, 0); // Turn the LED off, makes it clear the device was unloaded
gpio_unexport(gpioLED); // Unexport the LED GPIO
free_irq(irqNumber, NULL); // Free the IRQ number, no *dev_id required in this case
gpio_unexport(gpioButton); // Unexport the Button GPIO
gpio_free(gpioLED); // Free the LED GPIO
gpio_free(gpioButton); // Free the Button GPIO
printk(KERN_INFO "GPIO_TEST: Goodbye from the LKM!\n");
/** @brief The GPIO IRQ Handler function
* This function is a custom interrupt handler that is attached to the GPIO above. The same interrupt
* handler cannot be invoked concurrently as the interrupt line is masked out until the function is complete.
* This function is static as it should not be invoked directly from outside of this file.
* @param irq the IRQ number that is associated with the GPIO -- useful for logging.
* @param dev_id the *dev_id that is provided -- can be used to identify which device caused the interrupt
* Not used in this example as NULL is passed.
* @param regs h/w specific register values -- only really ever used for debugging.
* return returns IRQ_HANDLED if successful -- should return IRQ_NONE otherwise.
static irq_handler_t ebbgpio_irq_handler(unsigned int irq, void *dev_id, struct pt_regs *regs){
ledOn = !ledOn; // Invert the LED state on each button press
gpio_set_value(gpioLED, ledOn); // Set the physical LED accordingly
printk(KERN_INFO "GPIO_TEST: Interrupt! (button state is %d)\n", gpio_get_value(gpioButton));
numberPresses++; // Global counter, will be outputted when the module is unloaded
return (irq_handler_t) IRQ_HANDLED; // Announce that the IRQ has been handled correctly
/// This next calls are mandatory -- they identify the initialization function
/// and the cleanup function (as above).
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