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

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Date:   Tue, 9 Jul 2019 16:20:24 +0200
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Linus Walleij <linus.walleij@...aro.org>
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

wt., 9 lip 2019 o 15:30 Linus Walleij <linus.walleij@...aro.org> napisał(a):
>
> 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).
>

In this case the board doesn't provide any GPIO controller at all so
there's simply no need to select GPIOLIB - it would only add bloat.

> I was thinking something like this in the stubs:
>
> gpiod_get[_index]() {
>     return POISON;
> }
>
> gpiod_get[_index]_optional() {
>    return NULL;
> }

This is already being done.

>
> 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.)
>

Maybe I'm missing something, but I don't quite see how my patch
violates this behavior. :(

> 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

Bart