When initially designing this interface, I did not include these options as they are not available on all platforms and as I wanted to keep out everything that might bloat this interface (to keep it as slim and easy to use as possible). But there were also some earlier discussions with @PeterKietzmann about integrating this.

Now that I see it, most MCUs actually let you choose at least between push-pull and open-drain when in output mode. Tristate I have actually not yet seen (though I might not have looked very hard). In the contrary to the pull resistor selection, these modes are only applicable when in output mode.

So the question is how we could integrate this so it doesn't bloat the interface and so that it can be very efficiently implemented. I see these options:
1a. Add this as parameter to the init function:

int gpio_init(gpio_t pin, gpio_dir_t dir, gpio_omode_t omode, gpio_pp_t pull);
with
typedef enum {
    GPIO_OMODE_PUSHPULL,
    GPIO_OMODE_OPENDRAIN,
    GPIO_OMODE_TRISTATE,
} gpio_omode_t;

1b. Integrate these options into the dir parameter:

typedef enum {
    GPIO_DIR_IN = 0,    
    GPIO_DIR_OUT_PP,
    GPIO_DIR_OUT_OD,
    GPIO_DIR_OUT_TS,
} gpio_dir_t;

Add a special function for this and document that this function should only be used when pin is configured to be output

void gpio_outmode(gpio_t pin, gpio_omode_t mode);

Looking at this I would probably prefer option 1b, though it might take some beauty from some gpio initialization code...

@kaspar030, @gebart, @jfischer-phytec-iot, @PeterKietzmann: what are your opinions on this?

enhance gpio periphal driver #4472

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