3.4. Char Device Registration

As we mentioned, the kernel uses structures of type struct cdev to represent char devices internally. Before the kernel invokes your device's operations, you must allocate and register one or more of these structures.^[6] To do so, your code should include <linux/cdev.h>, where the structure and its associated helper functions are defined.

^[6] There is an older mechanism that avoids the use of cdev structures (which we discuss in Section 3.4.2). New code should use the newer technique, however.

There are two ways of allocating and initializing one of these structures. If you wish to obtain a standalone cdev structure at runtime, you may do so with code such as:

struct cdev *my_cdev = cdev_alloc(  );
my_cdev->ops = &my_fops;

Chances are, however, that you will want to embed the cdev structure within a device-specific structure of your own; that is what scull does. In that case, you should initialize the structure that you have already allocated with:

void cdev_init(struct cdev *cdev, struct file_operations *fops);

Either way, there is one other struct cdev field that you need to initialize. Like the file_operations structure, struct cdev has an owner field that should be set to THIS_MODULE.

Once the cdev structure is set up, the final step is to tell the kernel about it with a call to:

int cdev_add(struct cdev *dev, dev_t num, unsigned int count);

Here, dev is the cdev structure, num is the first device number to which this device responds, and count is the number of device numbers that should be associated with the device. Often count is one, but there are situations where it makes sense to have more than one device number correspond to a specific device. Consider, for example, the SCSI tape driver, which allows user space to select operating modes (such as density) by assigning multiple minor numbers to each physical device.

There are a couple of important things to keep in mind when using cdev_add. The first is that this call can fail. If it returns a negative error code, your device has not been added to the system. It almost always succeeds, however, and that brings up the other point: as soon as cdev_add returns, your device is "live" and its operations can be called by the kernel. You should not call cdev_add until your driver is completely ready to handle operations on the device.

To remove a char device from the system, call:

void cdev_del(struct cdev *dev);

Clearly, you should not access the cdev structure after passing it to cdev_del.

3.4.1. Device Registration in scull

Internally, scull represents each device with a structure of type struct scull_dev. This structure is defined as:

struct scull_dev {
    struct scull_qset *data;  /* Pointer to first quantum set */
    int quantum;              /* the current quantum size */
    int qset;                 /* the current array size */
    unsigned long size;       /* amount of data stored here */
    unsigned int access_key;  /* used by sculluid and scullpriv */
    struct semaphore sem;     /* mutual exclusion semaphore     */
    struct cdev cdev;     /* Char device structure      */
};

We discuss the various fields in this structure as we come to them, but for now, we call attention to cdev, the struct cdev that interfaces our device to the kernel. This structure must be initialized and added to the system as described above; the scull code that handles this task is:

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;
    err = cdev_add (&dev->cdev, devno, 1);
    /* Fail gracefully if need be */
    if (err)
    printk(KERN_NOTICE "Error %d adding scull%d", err, index);
}

Since the cdev structure is embedded within struct scull_dev, cdev_init must be called to perform the initialization of that structure.

3.4.2. The Older Way

If you dig through much driver code in the 2.6 kernel, you may notice that quite a few char drivers do not use the cdev interface that we have just described. What you are seeing is older code that has not yet been upgraded to the 2.6 interface. Since that code works as it is, this upgrade may not happen for a long time. For completeness, we describe the older char device registration interface, but new code should not use it; this mechanism will likely go away in a future kernel.

The classic way to register a char device driver is with:

int register_chrdev(unsigned int major, const char *name,
                    struct file_operations *fops);

Here, major is the major number of interest, name is the name of the driver (it appears in /proc/devices), and fops is the default file_operations structure. A call to register_chrdev registers minor numbers 0-255 for the given major, and sets up a default cdev structure for each. Drivers using this interface must be prepared to handle open calls on all 256 minor numbers (whether they correspond to real devices or not), and they cannot use major or minor numbers greater than 255.

If you use register_chrdev, the proper function to remove your device(s) from the system is:

int unregister_chrdev(unsigned int major, const char *name);

major and name must be the same as those passed to register_chrdev, or the call will fail.