How kernel derive irq number for particular irq line

Hi Guys, I want to know how the irq number is derived in linux kernel. In kernel we are using below function for registering ISR. int *request_threaded_irq*(unsigned int irq, irq_handler_t handler, irq_handler_t thread_fn, unsigned long irqflags, const char *devname, void *dev_id) 1st arg is irq number. Now suppose for an i2c device, want to configure an interrupt then we can enable interrupt by configuring the particular register but from where we will get the irq number details. Do we as a programmer need to map it ? Please provide your views. BR,Abhinav