Drivers for such devices should provide:

• Methods to establish GPIO direction (input and output).
• Methods used to access GPIO values (get and set).
• Methods to map a given GPIO to IRQ and return the associated number.
• Flag saying whether calls to its methods may sleep. This is very important.
• An optional debugfs dump method (showing extra state such as pullup config).
• An optional number called a base number, from which GPIO numbering should start. It will be automatically assigned if omitted.

In the kernel, a GPIO controller is represented as an instance of struct gpio_chip, defined in linux/gpio/driver.h:

struct gpio_chip { 
  const char *label; 
  struct device *dev; 
  struct module *owner; 
 
  int (*request)(struct gpio_chip *chip, unsigned offset); 
  void (*free)(struct gpio_chip *chip, unsigned offset); 
  int (*get_direction)(struct...