So, this is the API we are going to use

No specific header to include
The gpio header is included by default by cpu.h

GPIO_NR(port, bit)
GPIO_PORT(nr)
GPIO_BIT(nr)

extern void gpio_init(void);

extern int gpio_dir_af(int gpio, int output, int value, int afnum);
extern void gpio_dir(int gpio, int output, int value);

extern int gpio_get(int gpio);
extern uint32_t __gpio_get(int gpio);
extern void gpio_set(int gpio, int value);
extern void __gpio_set(int gpio, uint32_t value);

Then, there are constants to help the caller
GPIO_DIR_IN, GPIO_DIR_OUT, ...
Initialization can be slow (who cares)
Runtime may need to be fast
Sometimes, the program may directly act on registers