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This may be a novice question, but please help me to understand it.
Why exactly do we need to register the struct cdev in our character driver?
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struct cdev is the kernel's internal structure that represents char devices; this field contains a pointer to that structure when the inode refers to a char device file.
The file_operations Structure. The file_operations structure is defined in linux/fs. h, and holds pointers to functions defined by the driver that perform various operations on the device. Each field of the structure corresponds to the address of some function defined by the driver to handle a requested operation.
Dynamically Allocatingdev is an output-only parameter that will, on successful completion, hold the first number in your allocated range. firstminor should be the requested first minor number to use; it is usually 0. count is the total number of contiguous device numbers you are requesting.
cdev_alloc() dynamically allocates my_dev , so it will call kfree(pointer) when cdev_del() . cdev_init() will not free the pointer. Most importantly, the lifetime of the structure my_cdev is different.
struct cdev represent a character device within the kernel.
All streaming devices(ex: uart, keyboard) falls under the character device category and is available in user-space as a device node file (ex: /dev/ttyS0). User applications accesses the device by using the standard File I/O operations.
Inside kernel, character driver comes in between the device file and the streaming device, and this driver layer takes care of translating the File I/O operation to Device operations and vice-versa.
In character device driver development struct file_operations is the most important data structure used. This structure is used to implement the basic file i/o - open(), read(), write(), close(), ioctl(), etc... functionality for the the device.
struct cdev structure encapsulates the file_operations structure and some other important driver information(major/minor no). It is the new way of registering character driver with kernel.
The cdev structure is accessed by the kernel through the following API's:
cdev_init() - used to initialize struct cdev with the defined file_operations
cdev_add() - used to add a character device to the system.
cdev_del() - used to remove a character device from the system
After a call to cdev_add(), your device is immediately alive. All functions you defined (through the file_operations structure) can be called.
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