3.5. open and release

Now that we've taken a quick look at the fields, we start using them in real scull functions.

3.5.1. The open Method

The open method is provided for a driver to do any initialization in preparation for later operations. In most drivers, open should perform the following tasks:

• Check for device-specific errors (such as device-not-ready or similar hardware problems)

• Initialize the device if it is being opened for the first time

• Update the f_op pointer, if necessary

• Allocate and fill any data structure to be put in filp->private_data

The first order of business, however, is usually to identify which device is being opened. Remember that the prototype for the open method is:

int (*open)(struct inode *inode, struct file *filp);

The inode argument has the information we need in the form of its i_cdev field, which contains the cdev structure we set up before. The only problem is that we do not normally want the cdev structure itself, we want the scull_dev structure that contains that cdev structure. The C language lets programmers play all sorts of tricks to make that kind of conversion; programming such tricks is error prone, however, and leads to code that is difficult for others to read and understand. Fortunately, in this case, the kernel hackers have done the tricky stuff for us, in the form of the container_of macro, defined in <linux/kernel.h>:

container_of(pointer, container_type, container_field);

This macro takes a pointer to a field of type container_field, within a structure of type container_type, and returns a pointer to the containing structure. In scull_open, this macro is used to find the appropriate device structure:

struct scull_dev *dev; /* device information */

dev = container_of(inode->i_cdev, struct scull_dev, cdev);
filp->private_data = dev; /* for other methods */

Once it has found the scull_dev structure, scull stores a pointer to it in the private_data field of the file structure for easier access in the future.

The other way to identify the device being opened is to look at the minor number stored in the inode structure. If you register your device with register_chrdev, you must use this technique. Be sure to use iminor to obtain the minor number from the inode structure, and make sure that it corresponds to a device that your driver is actually prepared to handle.

The (slightly simplified) code for scull_open is:

int scull_open(struct inode *inode, struct file *filp)
{
    struct scull_dev *dev; /* device information */

    dev = container_of(inode->i_cdev, struct scull_dev, cdev);
    filp->private_data = dev; /* for other methods */

    /* now trim to 0 the length of the device if open was write-only */
    if ( (filp->f_flags & O_ACCMODE) =  = O_WRONLY) {
        scull_trim(dev); /* ignore errors */
    }
    return 0;          /* success */
}

The code looks pretty sparse, because it doesn't do any particular device handling when open is called. It doesn't need to, because the scull device is global and persistent by design. Specifically, there's no action such as "initializing the device on first open," because we don't keep an open count for sculls.

The only real operation performed on the device is truncating it to a length of 0 when the device is opened for writing. This is performed because, by design, overwriting a scull device with a shorter file results in a shorter device data area. This is similar to the way opening a regular file for writing truncates it to zero length. The operation does nothing if the device is opened for reading.

We'll see later how a real initialization works when we look at the code for the other scull personalities.

3.5.2. The release Method

The role of the release method is the reverse of open. Sometimes you'll find that the method implementation is called device_close instead of device_release. Either way, the device method should perform the following tasks:

• Deallocate anything that open allocated in filp->private_data

• Shut down the device on last close

The basic form of scull has no hardware to shut down, so the code required is minimal:^[7]

^[7] The other flavors of the device are closed by different functions because scull_open substituted a different filp->f_op for each device. We'll discuss these as we introduce each flavor.

int scull_release(struct inode *inode, struct file *filp)
{
    return 0;
}

You may be wondering what happens when a device file is closed more times than it is opened. After all, the dup and fork system calls create copies of open files without calling open; each of those copies is then closed at program termination. For example, most programs don't open their stdin file (or device), but all of them end up closing it. How does a driver know when an open device file has really been closed?

The answer is simple: not every close system call causes the release method to be invoked. Only the calls that actually release the device data structure invoke the method—hence its name. The kernel keeps a counter of how many times a file structure is being used. Neither fork nor dup creates a new file structure (only open does that); they just increment the counter in the existing structure. The close system call executes the release method only when the counter for the file structure drops to 0, which happens when the structure is destroyed. This relationship between the release method and the close system call guarantees that your driver sees only one release call for each open.

Note that the flush method is called every time an application calls close. However, very few drivers implement flush, because usually there's nothing to perform at close time unless release is involved.

As you may imagine, the previous discussion applies even when the application terminates without explicitly closing its open files: the kernel automatically closes any file at process exit time by internally using the close system call.