linux-kernel - Re: [PATCH 2/4] gpiolib: use const parameters when possible

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Message-ID: <20141117090915.GE27002@pengutronix.de>
Date:	Mon, 17 Nov 2014 10:09:15 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Alexandre Courbot <gnurou@...il.com>
Cc:	Grant Likely <grant.likely@...retlab.ca>,
	Linus Walleij <linus.walleij@...aro.org>,
	linux-kernel@...r.kernel.org,
	Alexandre Courbot <acourbot@...dia.com>
Subject: Re: [PATCH 2/4] gpiolib: use const parameters when possible

Hello,

On Wed, Feb 13, 2013 at 04:03:00PM +0900, Alexandre Courbot wrote:
> From: Alexandre Courbot <acourbot@...dia.com>
> 
> Constify descriptor parameter of gpiod_* functions for those that
> should obviously not modify it. This includes value or direction get,
> cansleep, and IRQ number query.
> 
> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
This patch is applied as def634338d3ffb32fbe9b0a2d70cc24ef909cd4f since
v3.9-rc2.

>  drivers/gpio/gpiolib.c | 29 ++++++++++++++++-------------
>  1 file changed, 16 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
> index 8a2cf9c..ad6df6b 100644
> --- a/drivers/gpio/gpiolib.c
> +++ b/drivers/gpio/gpiolib.c
> @@ -207,7 +208,7 @@ static int gpiochip_find_base(int ngpio)
>  }
>  
>  /* caller ensures gpio is valid and requested, chip->get_direction may sleep  */
> -static int gpiod_get_direction(struct gpio_desc *desc)
> +static int gpiod_get_direction(const struct gpio_desc *desc)
>  {
>  	struct gpio_chip	*chip;
>  	unsigned		offset;
> @@ -223,11 +224,13 @@ static int gpiod_get_direction(struct gpio_desc *desc)
>  	if (status > 0) {
>  		/* GPIOF_DIR_IN, or other positive */
>  		status = 1;
> -		clear_bit(FLAG_IS_OUT, &desc->flags);
> +		/* FLAG_IS_OUT is just a cache of the result of get_direction(),
> +		 * so it does not affect constness per se */
> +		clear_bit(FLAG_IS_OUT, &((struct gpio_desc *)desc)->flags);
I think this is worse than not having gpiod_get_direction take a const
descriptor.
I don't know an example for this particular case where this could break,
but usually casting away a const is bad and can break in various ways.

Maybe even drop the public function gpiod_get_direction completely.
As of next-20141114 there are only two users of this function (apart
from drivers/gpio/gpiolib*) and both are wrong (as discussed in
http://thread.gmane.org/gmane.linux.serial/16808/focus=4783).

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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