Interrupt request and propagation

Let us consider the following figure, which represents a chained IRQ flow:

Interrupt requests are always performed on Linux IRQ (not hwirq). The general function to request IRQ on Linux is request_threaded_irq() or request_irq(), which internally calls the former:

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) 

When called, the function extracts the struct irq_desc associated with the IRQ using the irq_to_desc() macro. It then allocates a new struct irqaction structure and sets it up, filling parameters such as the handler, flags, and so on.

action->handler = handler; 
action->thread_fn = thread_fn; 
action->flags = irqflags; 
action->name = devname; 
action->dev_id = dev_id; 

That same function...