linux-kernel - Re: [RFC PATCH] gpiolib: add gpio get direction support

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Message-ID: <CACRpkdbxfL+1NQ73ve8Hz4VKaVMzJS-P7H50kSNVODAU9mm7QA@mail.gmail.com>
Date:	Tue, 23 Oct 2012 00:23:49 +0200
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Mathias Nyman <mathias.nyman@...ux.intel.com>
Cc:	linus.walleij@...ricsson.com, grant.likely@...retlab.ca,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] gpiolib: add gpio get direction support

On Mon, Oct 22, 2012 at 3:44 PM, Mathias Nyman
<mathias.nyman@...ux.intel.com> wrote:

> Add gpio_get_direction() for checking the current direction of a gpio.
> Returns 1 for input, 0 for output, or negative error.
> Gpio drivers need to set the gpio_chip .get_direction callback for this
> functionality, otherwise gpio_get_direction() returns error.

OK...

> If the .get_direction callback is set, then gpiolib will use it
> for showing correct gpio direction in sysfs and debug. If not set
> then it will work the old way; e.g. guessing everything is input
> until direction is set.

If the driver cannot report the direction setting you *could*
cache the result of all calls to gpio_direction_output()
and gpio_direction_input() somewhere. But I don't know
if it's a good idea.

Anyway, if the callback is only called internally in the GPIOlib
why are you making the function public to the entire
kernel like this?

> include/asm-generic/gpio.h
> +extern int gpio_get_direction(unsigned gpio);

> include/linux/gpio.h
> +static inline int gpio_get_direction(unsigned gpio)
> +{
> +       return -ENOSYS;
> +}
> +

Yours,
Linus Walleij
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