linux-kernel - RE: [PATCH] floppy: fix harmless clang build warning

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Message-ID: <AT5PR8401MB116964D0C90783A164834C2CAB4A0@AT5PR8401MB1169.NAMPRD84.PROD.OUTLOOK.COM>
Date:   Wed, 13 Mar 2019 22:01:43 +0000
From:   "Elliott, Robert (Persistent Memory)" <elliott@....com>
To:     Arnd Bergmann <arnd@...db.de>, Jiri Kosina <jikos@...nel.org>,
        Jens Axboe <axboe@...nel.dk>
CC:     Omar Sandoval <osandov@...com>, Hannes Reinecke <hare@...e.de>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] floppy: fix harmless clang build warning



> -----Original Message-----
> From: linux-kernel-owner@...r.kernel.org [mailto:linux-kernel-owner@...r.kernel.org] On Behalf Of Arnd
> Bergmann
> Sent: Wednesday, March 13, 2019 4:11 PM
> To: Jiri Kosina <jikos@...nel.org>; Jens Axboe <axboe@...nel.dk>
> Cc: Arnd Bergmann <arnd@...db.de>; Omar Sandoval <osandov@...com>; Hannes Reinecke <hare@...e.de>;
> linux-block@...r.kernel.org; linux-kernel@...r.kernel.org
> Subject: [PATCH] floppy: fix harmless clang build warning
> 
> clang warns about unusual code in floppy.c that looks like it
> was intended to be a bit mask operation, checking for a specific
> bit in the UDP->cmos variable (FLOPPY1_TYPE expands to '4' on
> ARM):
> 
> drivers/block/floppy.c:3902:17: error: use of logical '&&' with constant operand [-Werror,-Wconstant-
> logical-operand]
>         if (!UDP->cmos && FLOPPY1_TYPE)
>                        ^  ~~~~~~~~~~~~
> drivers/block/floppy.c:3902:17: note: use '&' for a bitwise operation
>         if (!UDP->cmos && FLOPPY1_TYPE)
> 
> The check here is redundant anyway, if FLOPPY1_TYPE is zero, then
> assigning it to a zero UDP->cmos field does not change anything,
> so removing the extra check here has no effect other than shutting
> up the warning.
>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  drivers/block/floppy.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/block/floppy.c b/drivers/block/floppy.c
> index 95f608d1a098..56815446ed85 100644
> --- a/drivers/block/floppy.c
> +++ b/drivers/block/floppy.c
> @@ -3899,7 +3899,7 @@ static void __init config_types(void)
>  	if (!UDP->cmos)
>  		UDP->cmos = FLOPPY0_TYPE;
>  	drive = 1;
> -	if (!UDP->cmos && FLOPPY1_TYPE)
> +	if (!UDP->cmos)
>  		UDP->cmos = FLOPPY1_TYPE;
> 
>  	/* FIXME: additional physical CMOS drive detection should go here */
> --
> 2.20.0

On x86 it expands to a hardware read, so this would change the control flow.

#define FLOPPY1_TYPE                                    \
({                                                      \
        unsigned long flags;                            \
        unsigned char val;                              \
        spin_lock_irqsave(&rtc_lock, flags);            \
        val = CMOS_READ(0x10) & 15;                     \
        spin_unlock_irqrestore(&rtc_lock, flags);       \
        val;                                            \
})