linux-kernel - Re: [PATCH] spi-gpio: Sanitize MISO bitvalue

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Date:	Wed, 18 Feb 2009 13:04:26 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Michael Buesch <mb@...sch.de>
Cc:	dbrownell@...rs.sourceforge.net, linux-kernel@...r.kernel.org,
	openwrt-devel@...ts.openwrt.org
Subject: Re: [PATCH] spi-gpio: Sanitize MISO bitvalue

On Sun, 15 Feb 2009 16:30:41 +0100
Michael Buesch <mb@...sch.de> wrote:

> gpio_get_value() returns 0 or nonzero, but getmiso() expects 0 or 1.
> Sanitize the value to a 0/1 boolean.
> 
> Signed-off-by: Michael Buesch <mb@...sch.de>
> 
> ---
> 
> Well, we could also change the bitbang helpers in linux/spi/spi_bitbang.h
> or change the way the gpio_get_value API is defined, but I personally think
> this patch is pretty good as is.
> In any case, it fixes a real bug on platforms like the bcm47xx which
> return 0 or nonzero for gpio_get_value.
> 
> Index: linux-2.6/drivers/spi/spi_gpio.c
> ===================================================================
> --- linux-2.6.orig/drivers/spi/spi_gpio.c	2009-02-14 21:37:14.000000000 +0100
> +++ linux-2.6/drivers/spi/spi_gpio.c	2009-02-15 16:27:16.000000000 +0100
> @@ -114,7 +114,7 @@ static inline void setmosi(const struct 
>  
>  static inline int getmiso(const struct spi_device *spi)
>  {
> -	return gpio_get_value(SPI_MISO_GPIO);
> +	return !!gpio_get_value(SPI_MISO_GPIO);
>  }
>  
>  #undef pdata
> 

Seems somewhat pointless, really.  It's a very common C idiom to treat
any non-zero value as true, and the above just adds a couple more
instructions which we didn't need to execute.

If this function is speed-critical (which is what David's comment
implies) then perhaps this should be "fixed" by tightening up the
(presently apparently undocumented) interface?  And then speeding up
all the other getmiso() implementations?
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