Re: [PATCH 14/54] gpio: pch: Be sure to clamp return value

[Jean Delvare <jdelvare@xxxxxxx>]

Hi Linus,

On Tue, 22 Dec 2015 15:20:54 +0100, Linus Walleij wrote:
> As we want gpio_chip .get() calls to be able to return negative
> error codes and propagate to drivers, we need to go over all
> drivers and make sure their return values are clamped to [0,1].
> We do this by using the ret = !!(val) design pattern.
> 
> Cc: Thierry Reding <treding@xxxxxxxxxx>
> Cc: Daniel Krueger <daniel.krueger@xxxxxxxxxxxxxxxxxxxxx>
> Cc: Jean Delvare <jdelvare@xxxxxxx>
> Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
> ---
>  drivers/gpio/gpio-pch.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpio/gpio-pch.c b/drivers/gpio/gpio-pch.c
> index af0715f8524b..8c45b74dcf21 100644
> --- a/drivers/gpio/gpio-pch.c
> +++ b/drivers/gpio/gpio-pch.c
> @@ -127,7 +127,7 @@ static int pch_gpio_get(struct gpio_chip *gpio, unsigned nr)
>  {
>  	struct pch_gpio *chip =	container_of(gpio, struct pch_gpio, gpio);
>  
> -	return ioread32(&chip->reg->pi) & (1 << nr);
> +	return !!(ioread32(&chip->reg->pi) & (1 << nr));
>  }
>  
>  static int pch_gpio_direction_output(struct gpio_chip *gpio, unsigned nr,

I would prefer:

	return (ioread32(&chip->reg->pi) >> nr) & 1;

which is faster for the same result.

At x86 assembly level, your approach requires 5 CPU instructions (mov,
shl, test, setne and movzbl), mine only 2 CPU instructions (shr and
and.)

-- 
Jean Delvare
SUSE L3 Support
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