150 lines
4.2 KiB
C
150 lines
4.2 KiB
C
/*
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* chardev.c: Creates a read-only char device that says how many times
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* you have read from the dev file
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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/init.h>
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#include <linux/irq.h>
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#include <linux/kernel.h>
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#include <linux/module.h>
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#include <linux/poll.h>
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/* Prototypes - this would normally go in a .h file */
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static int device_open(struct inode *, struct file *);
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static int device_release(struct inode *, struct file *);
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static ssize_t device_read(struct file *, char *, size_t, loff_t *);
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static ssize_t device_write(struct file *, const char *, size_t, loff_t *);
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#define SUCCESS 0
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#define DEVICE_NAME "chardev" /* Dev name as it appears in /proc/devices */
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#define BUF_LEN 80 /* Max length of the message from the device */
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/* Global variables are declared as static, so are global within the file. */
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static int major; /* major number assigned to our device driver */
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static int open_device_cnt = 0; /* Is device open?
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* Used to prevent multiple access to device */
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static char msg[BUF_LEN]; /* The msg the device will give when asked */
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static char *msg_ptr;
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static struct class *cls;
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static struct file_operations chardev_fops = {
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.read = device_read,
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.write = device_write,
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.open = device_open,
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.release = device_release,
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};
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static int __init chardev_init(void)
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{
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major = register_chrdev(0, DEVICE_NAME, &chardev_fops);
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if (major < 0) {
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pr_alert("Registering char device failed with %d\n", major);
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return major;
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}
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pr_info("I was assigned major number %d.\n", major);
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cls = class_create(THIS_MODULE, DEVICE_NAME);
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device_create(cls, NULL, MKDEV(major, 0), NULL, DEVICE_NAME);
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pr_info("Device created on /dev/%s\n", DEVICE_NAME);
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return SUCCESS;
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}
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static void __exit chardev_exit(void)
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{
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device_destroy(cls, MKDEV(major, 0));
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class_destroy(cls);
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/* Unregister the device */
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unregister_chrdev(major, DEVICE_NAME);
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}
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/* Methods */
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/* Called when a process tries to open the device file, like
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* "sudo cat /dev/chardev"
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*/
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static int device_open(struct inode *inode, struct file *file)
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{
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static int counter = 0;
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if (open_device_cnt)
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return -EBUSY;
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open_device_cnt++;
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sprintf(msg, "I already told you %d times Hello world!\n", counter++);
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msg_ptr = msg;
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try_module_get(THIS_MODULE);
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return SUCCESS;
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}
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/* Called when a process closes the device file. */
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static int device_release(struct inode *inode, struct file *file)
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{
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open_device_cnt--; /* We're now ready for our next caller */
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/* Decrement the usage count, or else once you opened the file, you will
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* never get get rid of the module.
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*/
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module_put(THIS_MODULE);
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return SUCCESS;
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}
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/* Called when a process, which already opened the dev file, attempts to
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* read from it.
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*/
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static ssize_t device_read(struct file *filp, /* see include/linux/fs.h */
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char *buffer, /* buffer to fill with data */
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size_t length, /* length of the buffer */
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loff_t *offset)
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{
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/* Number of bytes actually written to the buffer */
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int bytes_read = 0;
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/* If we are at the end of message, return 0 signifying end of file. */
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if (*msg_ptr == 0)
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return 0;
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/* Actually put the data into the buffer */
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while (length && *msg_ptr) {
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/* The buffer is in the user data segment, not the kernel
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* segment so "*" assignment won't work. We have to use
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* put_user which copies data from the kernel data segment to
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* the user data segment.
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*/
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put_user(*(msg_ptr++), buffer++);
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length--;
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bytes_read++;
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}
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/* Most read functions return the number of bytes put into the buffer. */
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return bytes_read;
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}
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/* Called when a process writes to dev file: echo "hi" > /dev/hello */
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static ssize_t device_write(struct file *filp,
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const char *buff,
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size_t len,
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loff_t *off)
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{
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pr_alert("Sorry, this operation is not supported.\n");
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return -EINVAL;
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}
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module_init(chardev_init);
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module_exit(chardev_exit);
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MODULE_LICENSE("GPL");
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