linux-kernel - Re: [PATCH v2 21/23] gpio: protect the pointer to gpio_chip in gpio

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Message-ID: <CAMRc=MdRwCqn-A3Wdk8kaYDY6NAO5fC3wy0gxWsFQcPwoH9dvw@mail.gmail.com>
Date: Tue, 6 Feb 2024 14:23:35 +0100
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Linus Walleij <linus.walleij@...aro.org>, Kent Gibson <warthog618@...il.com>, 
	Alex Elder <elder@...aro.org>, Geert Uytterhoeven <geert+renesas@...der.be>, 
	"Paul E . McKenney" <paulmck@...nel.org>, Wolfram Sang <wsa@...-dreams.de>, linux-gpio@...r.kernel.org, 
	linux-kernel@...r.kernel.org, 
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v2 21/23] gpio: protect the pointer to gpio_chip in
 gpio_device with SRCU

On Tue, Feb 6, 2024 at 2:13 PM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> On Tue, Feb 06, 2024 at 01:57:39PM +0100, Bartosz Golaszewski wrote:
> > On Tue, Feb 6, 2024 at 1:24 PM Andy Shevchenko
> > <andriy.shevchenko@...ux.intel.com> wrote:
> > >
> > > On Mon, Feb 05, 2024 at 08:36:39PM +0100, Bartosz Golaszewski wrote:
> > > > On Mon, Feb 5, 2024 at 1:31 PM Andy Shevchenko
> > > > <andriy.shevchenko@...ux.intel.com> wrote:
> > >
> > > ...
> > >
> > > > >
> > > > > >  int gpiod_get_direction(struct gpio_desc *desc)
> > > > > >  {
> > > > > > -     struct gpio_chip *gc;
> > > > > >       unsigned long flags;
> > > > > >       unsigned int offset;
> > > > > >       int ret;
> > > > > >
> > > > > > -     gc = gpiod_to_chip(desc);
> > > > > > +     if (!desc)
> > > > > > +             /* Sane default is INPUT. */
> > > > > > +             return 1;
> > > > >
> > > > > Hmm... I can't imagine how this value may anyhow be used / useful.
> > > >
> > > > What else would you return for an optional (NULL) GPIO?
> > >
> > > An error. If somebody asks for direction of the non-existing GPIO, there is no
> > > (valid) answer for that.
>
> > All other functions return 0 for desc == NULL to accommodate
> > gpiod_get_optional(). I think we should stay consistent here.
>
> The way you proposed is inconsistent, i.e. you may not return any direction
> for the unknown / non-existing GPIO. You speculate it will be 1, I may consider
> that in my (hypothetical for now) case it should be 0.
>
> Just don't make all bananas to be oranges. It won't work.
>

I don't have a strong conviction here. May make it an error as well.
It's still inconsistent though - calling gpiod_direction_output(NULL);
will return 0 and then you get an error when you do
gpiod_get_direction(NULL). I don't have a good solution though.

Bart