linux-kernel - Re: [PATCH] gpio: aspeed: fix compile testing warning

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Date:   Mon, 09 Jul 2018 17:31:24 +0200
From:   Alexander Stein <alexander.stein@...tec-electronic.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Joel Stanley <joel@....id.au>,
        Andrew Jeffery <andrew@...id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Thierry Reding <treding@...dia.com>,
        linux-gpio@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-aspeed@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] gpio: aspeed: fix compile testing warning

On Monday, July 9, 2018, 4:56:03 PM CEST Arnd Bergmann wrote:
> Gcc cannot always see that BUG_ON(1) is guaranteed to not
> return, so we get a warning message in some configurations:
> 
> drivers/gpio/gpio-aspeed.c: In function 'bank_reg':
> drivers/gpio/gpio-aspeed.c:244:1: error: control reaches end of non-void
> function [-Werror=return-type]
> 
> Using a plain BUG() is easier here and avoids the problem.
> 
> Fixes: 44ddf559d579 ("gpio: aspeed: Rework register type accessors")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  drivers/gpio/gpio-aspeed.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpio/gpio-aspeed.c b/drivers/gpio/gpio-aspeed.c
> index 1e00f4045f9d..2342e154029b 100644
> --- a/drivers/gpio/gpio-aspeed.c
> +++ b/drivers/gpio/gpio-aspeed.c
> @@ -240,7 +240,7 @@ static inline void __iomem *bank_reg(struct aspeed_gpio
> *gpio, case reg_cmdsrc1:
>  		return gpio->base + bank->cmdsrc_regs + GPIO_CMDSRC_1;
>  	}
> -	BUG_ON(1);
> +	BUG();
>  }
> 
>  #define GPIO_BANK(x)	((x) >> 5)

Is the semantic of BUG() (and BUG_ON as well) to never return? If so, then 
just an idea: Is it possible to add some macro magic in BUG_ON(x) to fail 
compiling if x is compile-constant? Giving a hint the passed condition always 
fails, which indicates a problem, at least to me.
>From a short search I found this in drivers/gpu/vga/vgaarb.c L630-633:
>	if (vgadev_find(pdev) != NULL) {
>		BUG_ON(1);
>		goto fail;
>	}
You can't fail with a BUG_ON(1) and try to do some error handling after that.

Best regards,
Alexander