[PATCH] floppy: fix harmless clang build warning

[PATCH] floppy: fix harmless clang build warning

[Arnd Bergmann]

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@arndb.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

RE: [PATCH] floppy: fix harmless clang build warning

[Elliott, Robert (Persistent Memory)]

> -----Original Message-----
> From: linux-kernel-owner@vger.kernel.org [mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of Arnd
> Bergmann
> Sent: Wednesday, March 13, 2019 4:11 PM
> To: Jiri Kosina <jikos@kernel.org>; Jens Axboe <axboe@kernel.dk>
> Cc: Arnd Bergmann <arnd@arndb.de>; Omar Sandoval <osandov@fb.com>; Hannes Reinecke <hare@suse.de>;
> linux-block@vger.kernel.org; linux-kernel@vger.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@arndb.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;                                            \
})

Re: [PATCH] floppy: fix harmless clang build warning

[Keith Busch]

On Wed, Mar 13, 2019 at 10:01:43PM +0000, Elliott, Robert (Persistent Memory) wrote:
> > @@ -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;                                            \
> })

The end result is still the same, though. It just doesn't read the
hardware twice when FLOPPY1_TYPE is non-zero.