gpio_test.c File Reference

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. More...

#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/gpio.h>
#include <linux/interrupt.h>

Include dependency graph for gpio_test.c:

Functions
	MODULE_LICENSE ("GPL")
	MODULE_AUTHOR ("Derek Molloy")
	MODULE_DESCRIPTION ("A Button/LED test driver for the BBB")
	MODULE_VERSION ("0.1")
static irq_handler_t	ebbgpio_irq_handler (unsigned int irq, void *dev_id, struct pt_regs *regs)
	Function prototype for the custom IRQ handler function – see below for the implementation. More...
static int __init	ebbgpio_init (void)
	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. More...
static void __exit	ebbgpio_exit (void)
	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. More...
	module_init (ebbgpio_init)
	module_exit (ebbgpio_exit)

Variables
static unsigned int	gpioLED = 49
	hard coding the LED gpio for this example to P9_23 (GPIO49) More...
static unsigned int	gpioButton = 115
	hard coding the button gpio for this example to P9_27 (GPIO115) More...
static unsigned int	irqNumber
	Used to share the IRQ number within this file. More...
static unsigned int	numberPresses = 0
	For information, store the number of button presses. More...
static bool	ledOn = 0
	Is the LED on or off? Used to invert its state (off by default) More...

Detailed Description

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.

Author

Derek Molloy

Date

19 April 2015

See also

http://www.derekmolloy.ie/

Function Documentation

static void __exit ebbgpio_exit ( void  )

static

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.

                                     {
   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");
}

static int __init ebbgpio_init ( void  )

static

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.

Returns

returns 0 if successful

                                    {
   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;
}

static irq_handler_t ebbgpio_irq_handler ( unsigned int  irq, void *  dev_id, struct pt_regs *  regs  )

static

Function prototype for the custom IRQ handler function – see below for the implementation.

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.

Parameters

irq	the IRQ number that is associated with the GPIO – useful for logging.
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.
regs	h/w specific register values – only really ever used for debugging. return returns IRQ_HANDLED if successful – should return IRQ_NONE otherwise.

                                                                                              {
   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
}

MODULE_AUTHOR

(

"Derek Molloy"

)

MODULE_DESCRIPTION

(

"A Button/LED test driver for the BBB"

)

module_exit

(

ebbgpio_exit

)

module_init

(

ebbgpio_init

)

This next calls are mandatory – they identify the initialization function and the cleanup function (as above).

MODULE_LICENSE

(

"GPL"

)

MODULE_VERSION

(

"0.1"

)

Variable Documentation

unsigned int gpioButton = 115

static

hard coding the button gpio for this example to P9_27 (GPIO115)

unsigned int gpioLED = 49

static

hard coding the LED gpio for this example to P9_23 (GPIO49)

unsigned int irqNumber

static

Used to share the IRQ number within this file.

bool ledOn = 0

static

Is the LED on or off? Used to invert its state (off by default)

unsigned int numberPresses = 0

static

For information, store the number of button presses.