Why does module failed to load? (/dev/scull0 : no such device or address)

问题

From my previous question: How to build external modules in ubuntu? I thought I cannot load module due to loading out-of-tree or tainting kernel, but to my scull_init_module (source below) I have added printk so I know I get loaded from kern log. But I did not (no output in the log -- the log is the same as from the link).

uname -a == 4.19.0-9-amd64 #1 SMP Debian 4.19.118-2 (2020-04-29) x86_64 GNU/Linux (no use of ubuntu - as from the link).

scull_init_module:

#ifndef scull_major
#define scull_major 0
#endif

#ifndef scull_nr_devs
#define scull_nr_devs 4
#endif

struct scull_dev *scull_devices; /*array of active devices */
static void scull_setup_cdev(struct scull_dev *, int); /*helper function (internal) */  
int scull_minor = 0;


int scull_init_module(void) {
   int i;
   dev_t dev = 0; 
   /* register devices */
   int result = 1;
   result = alloc_chrdev_region(&dev, scull_minor, scull_nr_devs, "scull"); /**comment later**/
   printk(KERN_ALERT"Passed %s %d \n in scull_init_module\n", __FUNCTION__, __LINE__);

   if(result < 0){
      printk(KERN_WARNING "scull: can't get major %d\n", scull_major);
      return result;
   }

   /* allocate space for devices */ 
   scull_devices = kmalloc(sizeof(struct scull_dev) * scull_nr_devs, GFP_KERNEL);
   if(!scull_devices){
      result = -1;
      goto fail;
   }

   /*initilization of devices */
   for(i=0; i<scull_nr_devs;i++){
      scull_devices[i].quantum = scull_quantum;
      scull_devices[i].qset = scull_qset;
      scull_setup_cdev(&scull_devices[i], i);
   }

   return 0;
   fail:
      scull_cleanup_module();
      return result;
}

module_init(scull_init_module);

It is basically from Linux Device Drivers, Third Edition, chapter3. But I hand-write it (no copy/paste). alloc_chrdev_region is called properly, so is scull_setup_cdev, then where could be problem?

EDIT (output from dmesg): From dmesg it looks like it does load the module:

[ 2765.707018] scull: loading out-of-tree module taints kernel.
[ 2765.707106] scull: module verification failed: signature and/or required key missing - tainting kernel
[ 2765.707929] Passed scull_init_module 41 
                in scull_init_module

But then still the node in /dev/scull0 gives No such device or address

#mknod /dev/scull0 c 241 0
#cat /dev/scull0
cat: /dev/scull0: No such device or address

So if the device is properly registered (see dmesg) and I create node in /dev hierarchy, then what does mean no device or address? And why I am getting it?

EDIT2 (function scull_setup_cdev):

static void scull_setup_cdev(struct scull_dev *dev, int index){
    int err, devno=MKDEV(scull_major, scull_minor + index);

    cdev_init(&dev->cdev, &scull_fops);
    dev->cdev.owner = THIS_MODULE;
    dev->cdev.ops = &scull_fops; /**comment later**/
    err = cdev_add(&dev->cdev, devno, 1);   
    if (err) 
        printk(KERN_NOTICE "Error %d adding scull%d", err, index);
}

来源：https://stackoverflow.com/questions/62019834/why-does-module-failed-to-load-dev-scull0-no-such-device-or-address

标签

linux-device-driver

kernel-module

chardev