linux-kernel - Re: [PATCH 4/4] tty/serial: sh-sci: remove uneeded IS_ERR_OR

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Message-ID: <CAMuHMdXnUzjb5wzMtmzY2ojpGbhr_3KFGDYd2QKEawVipBhdbw@mail.gmail.com>
Date:   Fri, 24 Mar 2017 09:29:02 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Richard Genoud <richard.genoud@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Boris Brezillon <boris.brezillon@...e-electrons.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Janusz Uzycki <j.uzycki@...roma.com.pl>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH 4/4] tty/serial: sh-sci: remove uneeded IS_ERR_OR_NULL calls

Hi Uwe,

On Fri, Mar 24, 2017 at 9:00 AM, Uwe Kleine-König
<u.kleine-koenig@...gutronix.de> wrote:
> From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> Subject: [PATCH] gpiod: let get_optional return NULL in some cases with GPIOLIB disabled
>
> People disagree if gpiod_get_optional should return NULL or
> ERR_PTR(-ENOSYS) if GPIOLIB is disabled. The argument for NULL is that
> the person who decided to disable GPIOLIB is assumed to know that there
> is no GPIO. The reason to stick to ERR_PTR(-ENOSYS) is that it might
> introduce hard to debug problems if that decision is wrong.
>
> So this patch introduces a compromise and let gpiod_get_optional (and
> its variants) return NULL if the device in question cannot have an
> associated GPIO because it is neither instantiated by a device tree nor
> by ACPI.
>
> This should handle most cases that are argued about.
>
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> ---
>  include/linux/gpio/consumer.h | 55 ++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 46 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/gpio/consumer.h b/include/linux/gpio/consumer.h
> index fb0fde686cb1..0ca29889290d 100644
> --- a/include/linux/gpio/consumer.h
> +++ b/include/linux/gpio/consumer.h
> @@ -161,20 +161,48 @@ gpiod_get_index(struct device *dev,
>         return ERR_PTR(-ENOSYS);
>  }
>
> -static inline struct gpio_desc *__must_check
> -gpiod_get_optional(struct device *dev, const char *con_id,
> -                  enum gpiod_flags flags)
> +static inline bool __gpiod_no_optional_possible(struct device *dev)
>  {
> -       return ERR_PTR(-ENOSYS);
> +       /*
> +        * gpiod_get_optional et al can only provide a GPIO if at least one of
> +        * the backends for specifing a GPIO is available. These are device
> +        * tree, ACPI and gpiolib's lookup tables. The latter isn't available if
> +        * GPIOLIB is disabled (which is the case here).
> +        * So if the provided device is unrelated to device tree and ACPI, we
> +        * can be sure that there is no optional GPIO and let gpiod_get_optional
> +        * safely return NULL.
> +        * Otherwise there is still a chance that there is no GPIO but we cannot
> +        * be sure without having to enable a part of GPIOLIB (i.e. the lookup
> +        * part). So lets play safe and return an error. (Though there are also
> +        * arguments that returning NULL then would be beneficial.)
> +        */
> +
> +       if (IS_ENABLED(CONFIG_OF) && dev && dev->of_node)
> +               return false;

At first sight, I though this was OK:

  1. On ARM with DT, we can assume CONFIG_GPIOLOB=y.

  2. I managed to configure an SH kernel with CONFIG_GPIOLOB=n, CONFIG_OF=y,
     and CONFIG_SERIAL_SH_SCI=y, but since SH boards with SH-SCI UARTs do
     not use DT (yet), the check for dev->of_node (false) should handle
     that.

  3. However, I managed to do the same for h8300, which does use DT. Hence
     if mctrl_gpio would start relying on gpiod_get_optional(), this would
     break the sh-sci driver on h8300 :-(
     Note that h8300 doesn't have any GPIO drivers (yet?), so
     CONFIG_GPIPOLIB=n makes perfect sense!

So I'm afraid the only option is to always return NULL, and put the
responsability on the shoulders of the system integrator...

> +       if (IS_ENABLED(CONFIG_ACPI) && dev && ACPI_COMPANION(dev))
> +               return false;

No comments about the ACPI case.

>  static inline struct gpio_desc *__must_check
>  gpiod_get_index_optional(struct device *dev, const char *con_id,
>                          unsigned int index, enum gpiod_flags flags)
>  {
> +       if (__gpiod_no_optional_possible(dev))
> +               return NULL;
> +
>         return ERR_PTR(-ENOSYS);

Regardless of the above, given you use the exact same construct in four
locations, what about letting __gpiod_no_optional_possible() return the NULL
or ERR_PTR itself, and renaming it to e.g. __gpiod_no_optional_return_value()?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds