Long story short: the probe() function of the driver is called as a result of calling the register_driver for that specific bus. More precisely, it’s called by the probe() of that bus_type structure. In your case: i2c_bus_type.
Here’s the call chain in your I2C case:
- i2c_register_driver
- driver_register
- bus_add_driver
- driver_attach
- __driver_attach (for your device)
- driver_probe_device
- really_probe
- i2c_device_probe (this is what dev->bus->probe is for an i2c driver)
- your_probe_function