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Add an LED driver example using GPIO
Add an LED driver example using GPIO and add related knowledge. Test detail: - Tested on Raspberry Pi 5B with Raspberry Pi OS (Debian 12, Linux version 6.12.1-v8-16k+) - Verify that LED example compiles and loads successfully - Verify that LED turns on and off sucessfully
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‎.ci/non-working‎

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@@ -3,3 +3,4 @@ bh_threaded
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intrpt
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vkbd
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syscall-steal
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led

‎examples/Makefile‎

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@@ -31,6 +31,7 @@ obj-m += ioctl.o
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obj-m += vinput.o
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obj-m += vkbd.o
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obj-m += static_key.o
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obj-m += led.o
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PWD := $(CURDIR)
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‎examples/led.c‎

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/*
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* led.c - Using GPIO to control the LED on/off
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*/
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#include <linux/cdev.h>
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#include <linux/delay.h>
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#include <linux/device.h>
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#include <linux/fs.h>
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#include <linux/gpio.h>
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#include <linux/init.h>
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#include <linux/module.h>
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#include <linux/printk.h>
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#include <linux/types.h>
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#include <linux/uaccess.h>
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#include <linux/version.h>
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#include <asm/errno.h>
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#define SUCCESS 0
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#define DEVICE_NAME "gpio_led"
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/* The major device number. We can not rely on dynamic registration
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* any more.
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*/
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#define MAJOR_NUM 100
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#define BUF_LEN 2
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static char control_signal[BUF_LEN];
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static unsigned long device_buffer_size = 0;
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static struct class *cls;
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static struct device *dev;
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/* Define GPIOs for LEDs.
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* TODO: Change the numbers for the GPIO on your board.
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*/
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static struct gpio leds[] = { { 4, GPIOF_OUT_INIT_LOW, "LED 1" } };
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/* This is called whenever a process attempts to open the device file */
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static int device_open(struct inode *inode, struct file *file)
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{
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pr_info("device_open(%p)\n", file);
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try_module_get(THIS_MODULE);
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return SUCCESS;
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}
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static int device_release(struct inode *inode, struct file *file)
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{
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pr_info("device_release(%p,%p)\n", inode, file);
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module_put(THIS_MODULE);
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return SUCCESS;
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}
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/* called when somebody tries to write into our device file. */
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static ssize_t device_write(struct file *file, const char __user *buffer,
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size_t length, loff_t *offset)
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{
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pr_info("device_write(%p,%p,%ld)", file, buffer, length);
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device_buffer_size = min(BUF_LEN, length);
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if (copy_from_user(control_signal, buffer, device_buffer_size)) {
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return -EFAULT;
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}
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/* Determine the received signal to decide the LED on/off state. */
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switch (control_signal[0]) {
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case '0':
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gpio_set_value(leds[0].gpio, 0);
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pr_info("LED OFF");
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break;
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case '1':
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gpio_set_value(leds[0].gpio, 1);
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pr_info("LED ON");
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break;
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default:
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printk("Invalid value!\n");
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break;
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}
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*offset += device_buffer_size;
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/* Again, return the number of input characters used. */
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return device_buffer_size;
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}
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static struct file_operations fops = {
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.write = device_write,
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.open = device_open,
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.release = device_release, /* a.k.a. close */
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};
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/* Initialize the module - Register the character device */
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static int __init led_init(void)
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{
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int ret = 0;
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/* Register the character device (at least try) */
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ret = register_chrdev(MAJOR_NUM, DEVICE_NAME, &fops);
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/* Negative values signify an error */
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if (ret < 0) {
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pr_alert("%s failed with %d\n",
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"Sorry, registering the character device ", ret);
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return ret;
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}
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#if LINUX_VERSION_CODE >= KERNEL_VERSION(6, 4, 0)
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cls = class_create(DEVICE_NAME);
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#else
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cls = class_create(THIS_MODULE, DEVICE_NAME);
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#endif
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if (IS_ERR(cls)) {
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pr_err("Failed to create class for device\n");
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ret = PTR_ERR(cls);
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goto fail1;
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}
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dev = device_create(cls, NULL, MKDEV(MAJOR_NUM, 0), NULL, DEVICE_NAME);
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if (IS_ERR(dev)) {
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pr_err("Failed to create the device file\n");
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ret = PTR_ERR(dev);
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goto fail2;
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}
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pr_info("Device created on /dev/%s\n", DEVICE_NAME);
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ret = gpio_request(leds[0].gpio, leds[0].label);
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if (ret) {
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pr_err("Unable to request GPIOs for LEDs: %d\n", ret);
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goto fail3;
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}
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ret = gpio_direction_output(leds[0].gpio, leds[0].flags);
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if (ret) {
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pr_err("Failed to set GPIO %d direction\n", leds[0].gpio);
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goto fail4;
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}
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return 0;
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fail4:
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gpio_free(leds[0].gpio);
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fail3:
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device_destroy(cls, MKDEV(MAJOR_NUM, 0));
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fail2:
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class_destroy(cls);
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fail1:
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unregister_chrdev(MAJOR_NUM, DEVICE_NAME);
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return ret;
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}
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/* Cleanup - unregister the appropriate file from /proc */
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static void __exit led_exit(void)
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{
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gpio_set_value(leds[0].gpio, 0);
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gpio_free(leds[0].gpio);
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device_destroy(cls, MKDEV(MAJOR_NUM, 0));
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class_destroy(cls);
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unregister_chrdev(MAJOR_NUM, DEVICE_NAME);
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}
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module_init(led_init);
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module_exit(led_exit);
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MODULE_LICENSE("GPL");

‎lkmpg.tex‎

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@@ -1816,6 +1816,61 @@ \subsection{Flashing keyboard LEDs}
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Adding debug code can change the situation enough to make the bug seem to disappear.
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Thus, you should keep debug code to a minimum and make sure it does not show up in production code.
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\section{GPIO}
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\label{sec:gpio}
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\subsection{GPIO}
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\label{sec:gpio_introduction}
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General Purpose Input/Output (GPIO) appears on the development board as pins. It acts as a bridge for communication between the development board and external devices. You can think of it like a switch: users can turn it on or off (Input), and the development board can also turn it on or off (Output).
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To implement GPIO, you use the \cpp|gpio_request()| function to enable a specific GPIO pin. After successfully enabling it, you can check that the pin is being used by looking at /sys/kernel/debug/gpio.
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\begin{codebash}
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cat /sys/kernel/debug/gpio
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\end{codebash}
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There are other ways to register GPIOs. For example, you can use \cpp|gpio_request_one()| to register a GPIO while setting its direction (input or output) and initial state at the same time. You can also use \cpp|gpio_request_array()| to register multiple GPIOs at once. However, note that \cpp|gpio_request_array()| has been removed in Linux v6.10+.
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When using GPIO, you must set it as either output with \cpp|gpio_direction_output()| or \cpp|input with gpio_direction_input()|.
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\begin{itemize}
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\item when the GPIO is set as output, you can use \cpp|gpio_set_value()| to choose to set it to high voltage or low voltage.
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\item when the GPIO is set as input, you can use \cpp|gpio_get_value()| to read whether the voltage is high or low.
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\end{itemize}
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\subsection{Control the LED's on/off state}
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\label{sec:gpio_led}
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In Section \ref{sec:device_files}, we learned how to communicate with Device Files. Therefore, we will further use Device Files to control the LED on and off.
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In the implementation, a pull-down resistor is used. The positive electrode of the LED is connected to GPIO4, and the negative electrode is connected to GND. The materials used include a Raspberry Pi 5, an LED, single-core wires, and a 220Ω resistor.
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\samplec{examples/led.c}
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Make and install the module:
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\begin{codebash}
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make
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sudo insmod led.ko
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\end{codebash}
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Changes the permissions of a file:
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\begin{codebash}
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sudo chmod 666 > /dev/gpio_led
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\end{codebash}
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Switch on the LED:
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\begin{codebash}
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echo '1' > /dev/gpio_led
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\end{codebash}
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Switch off the LED:
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\begin{codebash}
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echo '0' > /dev/gpio_led
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\end{codebash}
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Finally, remove the test module:
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\begin{codebash}
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sudo rmmod led
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\end{codebash}
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\section{Scheduling Tasks}
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\label{sec:scheduling_tasks}
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There are two main ways of running tasks: tasklets and work queues.
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Pull requests are greatly appreciated.
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Happy hacking!
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\end{document}

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