linux-kernel - Re: [PATCH v2] ARM: pxa: fix GPIO double shifts

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Message-ID: <1469896245.3998.113.camel@perches.com>
Date:	Sat, 30 Jul 2016 09:30:45 -0700
From:	Joe Perches <joe@...ches.com>
To:	Robert Jarzmik <robert.jarzmik@...e.fr>,
	Daniel Mack <daniel@...que.org>,
	Haojian Zhuang <haojian.zhuang@...il.com>,
	Russell King <linux@...linux.org.uk>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] ARM: pxa: fix GPIO double shifts

On Sat, 2016-07-30 at 13:22 +0200, Robert Jarzmik wrote:
> The commit 9bf448c66d4b ("ARM: pxa: use generic gpio operation instead of
> gpio register") from Oct 17, 2011, leads to the following static checker
> warning:
>   arch/arm/mach-pxa/spitz_pm.c:172 spitz_charger_wakeup()
>   warn: double left shift '!gpio_get_value(SPITZ_GPIO_KEY_INT)
>         << (1 << ((SPITZ_GPIO_KEY_INT) & 31))'
> 
> As Dan reported, the value is shifted three times :
>  - once by gpio_get_value(), which returns either 0 or BIT(gpio)
>  - once by the shift operation '<<'
>  - a last time by GPIO_bit(gpio) which is BIT(gpio)
> 
> Therefore the calculation lead to a chained or operator of :
>  - (1 << gpio) << (1 << gpio) = (2^gpio)^gpio = 2 ^ (gpio * gpio)
> 
> It is be sheer luck the former statement works, only because each gpio
> used is strictly smaller than 6, and therefore 2^(gpio^2) never
> overflows a 32 bits value, and because it is used as a boolean value to
> check a gpio activation.

It may be better to change the charger_wakeup callback return
value from unsigned long to bool and modify the other use in
spitz_pm.c 

$ git grep -w charger_wakeup
arch/arm/mach-pxa/corgi_pm.c:   .charger_wakeup  = corgi_charger_wakeup,
arch/arm/mach-pxa/sharpsl_pm.c:                 if (sharpsl_pm.machinfo->charger_wakeup() != 0)
arch/arm/mach-pxa/sharpsl_pm.c:         if (sharpsl_pm.machinfo->charger_wakeup())
arch/arm/mach-pxa/sharpsl_pm.h: unsigned long (*charger_wakeup)(void);
arch/arm/mach-pxa/spitz_pm.c:   .charger_wakeup   = spitz_charger_wakeup,

> Fixes: 9bf448c66d4b ("ARM: pxa: use generic gpio operation instead of
> gpio register")
> Reported-by: Dan Carpenter <dan.carpenter@...cle.com>
> Signed-off-by: Robert Jarzmik <robert.jarzmik@...e.fr>
> ---
> Since v1: replaced binary ORs with logical ORs after assembly comparison
> ---
>  arch/arm/mach-pxa/corgi_pm.c | 8 +++-----
>  arch/arm/mach-pxa/spitz_pm.c | 5 ++---
>  2 files changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm/mach-pxa/corgi_pm.c b/arch/arm/mach-pxa/corgi_pm.c
> index d9206811be9b..8dc39d602884 100644
> --- a/arch/arm/mach-pxa/corgi_pm.c
> +++ b/arch/arm/mach-pxa/corgi_pm.c
> @@ -135,11 +135,9 @@ static unsigned long corgi_charger_wakeup(void)
>  {
>  	unsigned long ret;
>  
> -	ret = (!gpio_get_value(CORGI_GPIO_AC_IN) << GPIO_bit(CORGI_GPIO_AC_IN))
> -		| (!gpio_get_value(CORGI_GPIO_KEY_INT)
> -		<< GPIO_bit(CORGI_GPIO_KEY_INT))
> -		| (!gpio_get_value(CORGI_GPIO_WAKEUP)
> -		<< GPIO_bit(CORGI_GPIO_WAKEUP));
> +	ret = !gpio_get_value(CORGI_GPIO_AC_IN)
> +		|| !gpio_get_value(CORGI_GPIO_KEY_INT)
> +		|| !gpio_get_value(CORGI_GPIO_WAKEUP);
>  	return ret;
>  }
>  
> diff --git a/arch/arm/mach-pxa/spitz_pm.c b/arch/arm/mach-pxa/spitz_pm.c
> index ea9f9034cb54..dd85869f6f99 100644
> --- a/arch/arm/mach-pxa/spitz_pm.c
> +++ b/arch/arm/mach-pxa/spitz_pm.c
> @@ -168,9 +168,8 @@ static int spitz_should_wakeup(unsigned int resume_on_alarm)
>  static unsigned long spitz_charger_wakeup(void)
>  {
>  	unsigned long ret;
> -	ret = ((!gpio_get_value(SPITZ_GPIO_KEY_INT)
> -		<< GPIO_bit(SPITZ_GPIO_KEY_INT))
> -		| gpio_get_value(SPITZ_GPIO_SYNC));
> +	ret = !gpio_get_value(SPITZ_GPIO_KEY_INT)
> +		|| gpio_get_value(SPITZ_GPIO_SYNC);
>  	return ret;
>  }
>