Counter

Makefile

@@ -1,4 +1,4 @@
-obj-m += hello.o
+obj-m += counter.o
 # make -C $(nix-build -E '(import <nixpkgs> {}).linux.dev' --no-out-link)/lib/modules/*/build M=$(pwd)
 
 all:

counter.c

@@ -0,0 +1,133 @@
+#include <linux/cdev.h>
+#include <linux/delay.h>
+#include <linux/device.h>
+#include <linux/fs.h>
+#include <linux/init.h>
+#include <linux/irq.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/poll.h>
+#include <linux/kthread.h>
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Inex Code");
+MODULE_DESCRIPTION("A counter device");
+MODULE_VERSION("0.0.3");
+
+#define DEVICE_NAME "inex_counter"
+
+static int device_open(struct inode *, struct file *);
+static int device_release(struct inode *, struct file *);
+static ssize_t device_read(struct file *, char __user *, size_t, loff_t *);
+static ssize_t device_write(struct file *, const char __user *, size_t, loff_t *);
+
+static struct file_operations counter_fops = {
+    .read = device_read,
+    .write = device_write,
+    .open = device_open,
+    .release = device_release,
+};
+
+enum {
+    CDEV_NOT_USED = 0,
+    CDEV_EXCLUSIVE_OPEN = 1,
+};
+
+static atomic_t already_open = ATOMIC_INIT(CDEV_NOT_USED);
+
+static int Major;
+
+static struct class *cls;
+
+static int counter = 0;
+static int is_read = 0;
+
+struct task_struct *ts;
+
+static int counter_thread(void *data) {
+	while (!kthread_should_stop()) {
+		counter++;
+		is_read = 0;
+		msleep(5000);
+	}
+	return 0;
+}
+
+static int __init counter_init(void) {
+	printk(KERN_INFO "Counter: Initializing the counter device.\n");
+	Major = register_chrdev(0, DEVICE_NAME, &counter_fops);
+	if (Major < 0) {
+		printk(KERN_ALERT "Registering char device failed with %d\n", Major);
+		return Major;
+	}
+	printk(KERN_INFO "I was assigned major number %d.", Major);
+
+	cls = class_create(THIS_MODULE, DEVICE_NAME);
+	device_create(cls, NULL, MKDEV(Major, 0), NULL, DEVICE_NAME);
+
+	ts = kthread_run(counter_thread, NULL, "counter_thread");
+	return 0;
+}
+
+static void __exit counter_goodbye(void) {
+	kthread_stop(ts);
+	device_destroy(cls, MKDEV(Major, 0));
+	class_destroy(cls);
+	unregister_chrdev(Major, "inex_counter");
+	printk(KERN_INFO "Counter: Goodbye cruel world.\n");
+}
+
+static int device_open(struct inode *inode, struct file *file) {
+	if (atomic_cmpxchg(&already_open, CDEV_NOT_USED, CDEV_EXCLUSIVE_OPEN)) {
+		return -EBUSY;
+	}
+
+	try_module_get(THIS_MODULE);
+	return 0;
+}
+
+static int device_release(struct inode *inode, struct file *file) {
+	atomic_set(&already_open, CDEV_NOT_USED);
+	module_put(THIS_MODULE);
+	is_read = 0;
+	return 0;
+}
+
+static ssize_t device_read(struct file *filp, char __user *buffer, size_t length, loff_t *offset) {
+	// Output the counter value to the user.
+	int bytes_read = 0;
+	char buf[32];
+	int len = sprintf(buf, "%d\n", counter);
+	if (is_read) {
+		return 0;
+	}
+	if (len > length) {
+		return -EINVAL;
+	}
+	if (copy_to_user(buffer, buf, len)) {
+		return -EINVAL;
+	}
+	bytes_read = len;
+	printk("Counter: Read %d bytes, counter = %d", bytes_read, counter);
+	is_read = 1;
+	return bytes_read;
+}
+
+static ssize_t device_write(struct file *filp, const char __user *buffer, size_t length, loff_t *offset) {
+	// Input the counter value from the user.
+	char buf[32];
+	int len = length;
+	if (len > sizeof(buf)) {
+		len = sizeof(buf);
+	}
+	if (copy_from_user(buf, buffer, len)) {
+		return -EINVAL;
+	}
+	buf[len] = 0;
+	if (sscanf(buf, "%d", &counter) != 1) {
+		return -EINVAL;
+	}
+	return len;
+}
+
+module_init(counter_init);
+module_exit(counter_goodbye);