Date:   Fri, 24 Mar 2017 10:44:50 +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 10:15 AM, Uwe Kleine-König
<u.kleine-koenig@...gutronix.de> wrote:
> On Fri, Mar 24, 2017 at 09:59:04AM +0100, Geert Uytterhoeven wrote:
>> On Fri, Mar 24, 2017 at 9:39 AM, Uwe Kleine-König
>> <u.kleine-koenig@...gutronix.de> wrote:
>> > On Fri, Mar 24, 2017 at 09:29:02AM +0100, Geert Uytterhoeven wrote:
>> >> 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!
>> >
>> > Thanks for your efforts.
>>
>> You're welcome.
>>
>> >> So I'm afraid the only option is to always return NULL, and put the
>> >> responsability on the shoulders of the system integrator...
>> >
>> > The gpio lines could be provided by an i2c gpio adapter, right? So IMHO
>> > you don't need platform gpios to justify -ENODEV. So I guess that's a
>> > case where we don't come to an agreement.
>>
>> While you can enable I2C without further dependencies, no I2C GPIO expander
>> will be offered... unless you have enabled CONFIG_GPIOLIB first.
>
> And that is expected, still the device tree could reference such a GPIO
> and thus create a situation where Dmitry's and my judgement disagree.

If the device tree references such a GPIO, and CONFIG_GPIOLIB is not set,
the I2C GPIO expander device will not be bound.
Frank Rowand's DT scripts (http://elinux.org/Device_Tree_frowand) will come
to the rescue, and inform the user which driver(s) to enable.

> So I think my suggestion is the best we could do now. It minimizes the
> number of cases where we disagree. The next best thing would be to
> implement that half gpiolib stuff (i.e. do the full lookup to be sure
> there is no gpio) but this comes at a price: We need some time to
> implement it and it adds a bit to the kernel size.

So I still have to handle -ENOSYS in sh-sci.c, to avoid regressions...

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

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