Linux Device Drivers Development

The legacy GPIO and IRQ chip

1. The first step is to allocate a struct irq_domain to our gpiochip that will store the mapping between hwirq and virq. The linear mapping is suitable for us. We will do that in the probe function. That domain will hold the number of IRQs that our drivers wish to provide. For example, for an I/O expander, the number of IRQs could be the number of GPIOs the expander provides:

my_gpiochip->irq_domain = irq_domain_add_linear( client->dev.of_node, 
             my_gpiochip->chip.ngpio, &mcp23016_irq_domain_ops, NULL); 

The host_data parameter is NULL. Therefore, you can parse whatever data structure you need. Prior to allocating the domain, our domain ops structure should be defined, as follows:

static struct irq_domain_ops mcp23016_irq_domain_ops ...