Re: [PATCH 4/4] tty/serial: sh-sci: remove uneeded IS_ERR_OR_NULL calls

From: Geert Uytterhoeven
Date: Fri Mar 24 2017 - 04:30:02 EST


Hi Uwe,

On Fri, Mar 24, 2017 at 9:00 AM, Uwe Kleine-KÃnig
<u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
> From: Uwe Kleine-KÃnig <u.kleine-koenig@xxxxxxxxxxxxxx>
> 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-KÃnig <u.kleine-koenig@xxxxxxxxxxxxxx>
> ---
> 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@xxxxxxxxxxxxxx

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