linux-kernel - Re: [PATCH] gpio: don't WARN() on NULL descs if gpiolib is disabled

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Date:   Tue, 9 Jul 2019 15:30:22 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        "Claus H . Stovgaard" <cst@...seone.com>
Subject: Re: [PATCH] gpio: don't WARN() on NULL descs if gpiolib is disabled

Hi Bartosz,

On Mon, Jul 8, 2019 at 10:25 AM Bartosz Golaszewski <brgl@...ev.pl> wrote:

> From: Bartosz Golaszewski <bgolaszewski@...libre.com>
>
> If gpiolib is disabled, we use the inline stubs from gpio/consumer.h
> instead of regular definitions of GPIO API. The stubs for 'optional'
> variants of gpiod_get routines return NULL in this case as if the
> relevant GPIO wasn't found. This is correct so far.
>
> Calling other (non-gpio_get) stubs from this header triggers a warning
> because the GPIO descriptor couldn't have been requested. The warning
> however is unconditional (WARN_ON(1)) and is emitted even if the passed
> descriptor pointer is NULL.
>
> We don't want to force the users of 'optional' gpio_get to check the
> returned pointer before calling e.g. gpiod_set_value() so let's only
> WARN on non-NULL descriptors.
>
> Reported-by: Claus H. Stovgaard <cst@...seone.com>
> Signed-off-by: Bartosz Golaszewski <bgolaszewski@...libre.com>

I remember I had this discussion in the past, and I made a large
refactoring to make it possible for drivers that need gpiod_*
calls to simply do:

select GPIOLIB

in Kconfig.

This should solve also this problem I think.

However I do realize that there may be situations where people
simply want to make GPIO support entirely optional without
having to e.g. create custom stubs and encapsulate things
inside if IS_ENABLED(CONFIG_GPIOLIB).

I was thinking something like this in the stubs:

gpiod_get[_index]() {
    return POISON;
}

gpiod_get[_index]_optional() {
   return NULL;
}

This way all gpiod_get() and optional calls are properly
handled and the semantic that only _optional calls
can return NULL is preserved. (Your patch would
violate this.)

Then other stubs can do:

gpiod_set_value() {
  WARN_ON(desc);
}

As in your patch, and all will be smooth provided the
_optional calls have been used to obtain the desc.

Yours,
Linus Walleij