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

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Date:	Wed, 19 Nov 2014 17:57:46 +0900
From:	Alexandre Courbot <gnurou@...il.com>
To:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
Cc:	Grant Likely <grant.likely@...retlab.ca>,
	Linus Walleij <linus.walleij@...aro.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alexandre Courbot <acourbot@...dia.com>
Subject: Re: [PATCH 2/4] gpiolib: use const parameters when possible

On Wed, Nov 19, 2014 at 5:44 PM, Uwe Kleine-König
<u.kleine-koenig@...gutronix.de> wrote:
> Hello Alexandre,
>
> On Wed, Nov 19, 2014 at 05:33:42PM +0900, Alexandre Courbot wrote:
>> On Mon, Nov 17, 2014 at 6:09 PM, Uwe Kleine-König
>> <u.kleine-koenig@...gutronix.de> wrote:
>> > 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.
>>
>> Yeah, this kind of sucks but ultimately gpiod_get_direction() is
>> without side-effect to the caller and the const parameter helps
>> highlighting that fact.
> Does that mean you want to keep the const and take the risk that it
> might introduce some hard to debug problem when the compiler changes its
> optimisation strategies? I hope you don't.

Nope, I would like to keep that const but correctness is more
important than prototype aesthetics. I will fix that.

>> > 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).
>>
>> Or maybe gpiod_direction_*() should register the last set direction to
>> be used by gpiod_get_direction() if the controller does not implement
>> the get_direction op? On the other hand callers can also track the
>> direction themselves (since they presumably set it), so this function
>> is really only useful for users who want to read the *hardware* state
>> of a GPIO. Which might still be a useful feature?
> I think it's useful to be able to check how a given gpio is configured
> in hardware. So you really want to look into the register and not cache
> the direction in gpiod_direction_*.
> And I'd say having /sys/kernel/debug/gpio is good enough. So IMHO there
> is no reason to hide gpiod_get_direction from general misuse.

Mmm so are you suggesting that we should keep gpiod_get_direction() in the end?
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