Why is the probe method needed in Linux device drivers in addition to init?
In the linux kernel, what does the probe() method, that the driver provides, do? How different is it from the driver's init function, i.e. why can't the probe() functions actions be performed in the driver's init function ?
Answer
Different device types can have probe() functions. For example, PCI and USB devices both have probe() functions.
If you're talking about PCI devices, I would recommend you read chapter 12 of Linux Device Drivers, which covers this part of driver initialization. USB is covered in chapter 13.
Shorter answer, assuming PCI: The driver's init function calls pci_register_driver() which gives the kernel a list of devices it is able to service, along with a pointer to the probe() function. The kernel then calls the driver's probe() function once for each device.
This probe function starts the per-device initialization: initializing hardware, allocating resources, and registering the device with the kernel as a block or network device or whatever it is.
That makes it easier for device drivers, because they never need to search for devices or worry about finding a device that was hot-plugged. The kernel handles that part and notifies the right driver when it has a device for you to handle.