linux-kernel - Re: [PATCH] Define a NO_GPIO macro to compare against and to use as an invalid GPIO

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Date:	Fri, 8 Feb 2008 15:43:42 -0800
From:	David Brownell <david-b@...bell.net>
To:	Guennadi Liakhovetski <g.liakhovetski@...gutronix.de>
Cc:	linux-kernel@...r.kernel.org, i2c@...sensors.org
Subject: Re: [PATCH] Define a NO_GPIO macro to compare against and to use as an invalid GPIO

On Thursday 31 January 2008, Guennadi Liakhovetski wrote:
> As discussed on i2c mailing list with David Brownell, and number
> outside of the 0...MAX_INT range is invalid as a GPIO number.
> Define a macro, similar to NO_IRQ, to be used as a deliberate
> invalid GPIO, rather than defining a is_valid_gpio() macro.

Actually I thought that what you needed was an is_valid_gpio();
your motivation was that you needed a predicate.

The problem I have with a #define for a single such invalid GPIO
number is that people will inevitably start to assume it's the
only such number.  In particular "if (gpio == NO_GPIO) ..."
is by definition incorrect.

So I'd really rather see a predicate like is_valid_gpio().

If you want to designate one value for use as an initializer,
then I'd rather see a simple

	#define NO_GPIO	(-EINVAL)

without any option for arch-specific overrides ... along with a
comment that this is only *one* of the numerous values which
will fail is_valid_gpio().

- Dave



> 
> Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@...gutronix.de>
> 
> ---
> 
> As gpiolib doesn't seem to have an own mailing list, sending it directly 
> to LKML.
> 
>  include/asm-generic/gpio.h |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
> 
> diff --git a/include/asm-generic/gpio.h b/include/asm-generic/gpio.h
> index f29a502..806b86c 100644
> --- a/include/asm-generic/gpio.h
> +++ b/include/asm-generic/gpio.h
> @@ -16,6 +16,10 @@
>  #define ARCH_NR_GPIOS		256
>  #endif
>  
> +#ifndef NO_GPIO
> +#define NO_GPIO			((unsigned int)-1)
> +#endif
> +
>  struct seq_file;
>  
>  /**
> -- 
> 1.5.3.4
> 


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