linux-kernel - Re: [PATCH v4] ARM: pxa: fix GPIO double shifts

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Date:	Tue, 09 Aug 2016 20:52:17 +0200
From:	Robert Jarzmik <robert.jarzmik@...e.fr>
To:	Daniel Mack <daniel@...que.org>
Cc:	Haojian Zhuang <haojian.zhuang@...il.com>,
	Russell King <linux@...linux.org.uk>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Joe Perches <joe@...ches.com>
Subject: Re: [PATCH v4] ARM: pxa: fix GPIO double shifts

Robert Jarzmik <robert.jarzmik@...e.fr> writes:

> 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.
>
> As the xxx_charger_wakeup() functions are used as a true/false detection
> mechanism, take that opportunity to change their prototypes from integer
> return value to boolean one.
>
> Fixes: 9bf448c66d4b ("ARM: pxa: use generic gpio operation instead of
> gpio register")
> Reported-by: Dan Carpenter <dan.carpenter@...cle.com>
> Cc: Joe Perches <joe@...ches.com>
> Signed-off-by: Robert Jarzmik <robert.jarzmik@...e.fr>
Queued into pxa/for-next.

--
Robert