character_device_create

kernel_module/README.md

@@ -31,6 +31,7 @@
         1.  [dep](dep.c)
         1.  [dep2](dep2.c)
     1.  [character_device](character_device.c)
+        1.  [character_device_create](character_device_create.c)
     1.  [virt_to_phys](virt_to_phys.c)
     1.  Hardware device drivers
         1.  [pci_min](pci_min.c)

kernel_module/character_device.c

@@ -19,38 +19,31 @@ and use that for the mknod.
 - https://unix.stackexchange.com/questions/37829/understanding-character-device-or-character-special-files/371758#371758
 */
 
-#include <asm/uaccess.h> /* copy_from_user, copy_to_user */
-#include <linux/errno.h> /* EFAULT */
 #include <linux/fs.h> /* register_chrdev, unregister_chrdev */
-#include <linux/jiffies.h>
 #include <linux/module.h>
-#include <linux/printk.h> /* printk */
-#include <uapi/linux/stat.h> /* S_IRUSR */
+#include <linux/seq_file.h> /* seq_read, seq_lseek, single_release */
 
 #define NAME "lkmc_character_device"
 
 static int major;
 
-static ssize_t read(struct file *filp, char __user *buf, size_t len, loff_t *off)
+static int show(struct seq_file *m, void *v)
 {
-	size_t ret;
-	char kbuf[] = {'a', 'b', 'c', 'd'};
+	seq_printf(m, "abcd");
+	return 0;
+}
 
-	ret = 0;
-	if (*off == 0) {
-		if (copy_to_user(buf, kbuf, sizeof(kbuf))) {
-			ret = -EFAULT;
-		} else {
-			ret = sizeof(kbuf);
-			*off = 1;
-		}
-	}
-	return ret;
+static int open(struct inode *inode, struct  file *file)
+{
+	return single_open(file, show, NULL);
 }
 
 static const struct file_operations fops = {
+	.llseek = seq_lseek,
+	.open = open,
 	.owner = THIS_MODULE,
-	.read = read,
+	.read = seq_read,
+	.release = single_release,
 };
 
 static int myinit(void)

kernel_module/character_device_create.c

@@ -0,0 +1,61 @@
+/*
+Automatically create the device under /dev on insmod, and remove on rmmod.
+
+https://stackoverflow.com/questions/5970595/create-a-device-node-in-code/
+https://stackoverflow.com/questions/5970595/how-to-create-a-device-node-from-the-init-module-code-of-a-linux-kernel-module/18594761#18594761
+*/
+
+#include <linux/cdev.h>
+#include <linux/device.h>
+#include <linux/fs.h> /* register_chrdev, unregister_chrdev */
+#include <linux/module.h>
+#include <linux/seq_file.h> /* seq_read, seq_lseek, single_release */
+
+#define NAME "lkmc_character_device_create"
+
+static int major = -1;
+static struct cdev mycdev;
+static struct class *myclass = NULL;
+
+static int show(struct seq_file *m, void *v)
+{
+	seq_printf(m, "abcd");
+	return 0;
+}
+
+static int open(struct inode *inode, struct file *file)
+{
+	return single_open(file, show, NULL);
+}
+
+static const struct file_operations fops = {
+	.llseek = seq_lseek,
+	.open = open,
+	.owner = THIS_MODULE,
+	.read = seq_read,
+	.release = single_release,
+};
+
+static int myinit(void)
+{
+    /* cat /proc/devices */
+    alloc_chrdev_region(&major, 0, 1, NAME "_proc");
+    /* ls /sys/class */
+    myclass = class_create(THIS_MODULE, NAME "_sys");
+    /* ls /dev/ */
+    device_create(myclass, NULL, major, NULL, NAME "_dev");
+    cdev_init(&mycdev, &fops);
+    cdev_add(&mycdev, major, 1);
+    return 0;
+}
+
+static void myexit(void)
+{
+    device_destroy(myclass, major);
+    class_destroy(myclass);
+    unregister_chrdev_region(major, 1);
+}
+
+module_init(myinit)
+module_exit(myexit)
+MODULE_LICENSE("GPL");

rootfs_overlay/character_device.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
+set -ex
 insmod /character_device.ko
 /mknoddev.sh lkmc_character_device
-cat /dev/lkmc_character_device
-# => abcd
+[ "$(cat /dev/lkmc_character_device)" = 'abcd' ]
 rm /dev/lkmc_character_device
 rmmod character_device

rootfs_overlay/character_device_create.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+set -ex
+insmod /character_device_create.ko
+dev='/dev/lkmc_character_device_create_dev'
+[ "$(cat "$dev")" = 'abcd' ]
+rmmod character_device_create
+[ ! -e "$dev" ]