Warning with asm-generic/div64.h

Warning with asm-generic/div64.h

[Guan Xuetao]

Hi, Arnd:

Following warning is generated when using <asm-generic/div64.h>

kernel/power/hibernate.c: In function 'swsusp_show_speed':
kernel/power/hibernate.c:227: warning: comparison of distinct pointer types lacks a cast

After expanding the do_div() macro, the problem line is:

	(void)(((typeof((n)) *)0) == ((uint64_t *)0));

while the type of n is s64.

I am puzzled with the usage of the problem line.

And how to correct it?

Guan Xuetao

Re: Warning with asm-generic/div64.h

[Arnd Bergmann]

On Friday 25 March 2011, Guan Xuetao wrote:
> Following warning is generated when using <asm-generic/div64.h>
> 
> kernel/power/hibernate.c: In function 'swsusp_show_speed':
> kernel/power/hibernate.c:227: warning: comparison of distinct pointer types lacks a cast
> 
> After expanding the do_div() macro, the problem line is:
> 
>         (void)(((typeof((n)) *)0) == ((uint64_t *)0));
> 
> while the type of n is s64.
> 
> I am puzzled with the usage of the problem line.
> 
> And how to correct it?

The purpose of this line is to generate a warning when the input is the
wrong type, as the comment says:
 /* The unnecessary pointer compare is there
  * to check for type safety (n must be 64bit)

The macro doesn't work for signed input or for 32 bit input. I'd suggest
fixing the caller to use an unsigned type.

	Arnd

RE: Warning with asm-generic/div64.h

[Guan Xuetao]

> -----Original Message-----
> From: Arnd Bergmann [mailto:arnd@arndb.de]
> Sent: Friday, March 25, 2011 9:40 PM
> To: Guan Xuetao
> Cc: linux-kernel@vger.kernel.org; linux-arch@vger.kernel.org
> Subject: Re: Warning with asm-generic/div64.h
> 
> On Friday 25 March 2011, Guan Xuetao wrote:
> > Following warning is generated when using <asm-generic/div64.h>
> >
> > kernel/power/hibernate.c: In function 'swsusp_show_speed':
> > kernel/power/hibernate.c:227: warning: comparison of distinct pointer types lacks a cast
> >
> > After expanding the do_div() macro, the problem line is:
> >
> >         (void)(((typeof((n)) *)0) == ((uint64_t *)0));
> >
> > while the type of n is s64.
> >
> > I am puzzled with the usage of the problem line.
> >
> > And how to correct it?
> 
> The purpose of this line is to generate a warning when the input is the
> wrong type, as the comment says:
>  /* The unnecessary pointer compare is there
>   * to check for type safety (n must be 64bit)
Yes, it works. And, I think type s64 should be ok also.

> 
> The macro doesn't work for signed input or for 32 bit input. I'd suggest
> fixing the caller to use an unsigned type.

Then,
Cc: Len Brown <len.brown@intel.com> (supporter:SUSPEND TO RAM)
Cc: Pavel Machek <pavel@ucw.cz> (supporter:SUSPEND TO RAM)
Cc: "Rafael J. Wysocki" <rjw@sisk.pl> (supporter:SUSPEND TO RAM)
Cc: linux-pm@lists.linux-foundation.org (open list:SUSPEND TO RAM)

> 
> 	Arnd
Thanks, and sorry for replying late.

Guan Xuetao

Re: Warning with asm-generic/div64.h

[Arnd Bergmann]

On Tuesday 29 March 2011, Guan Xuetao wrote:
> > The macro doesn't work for signed input or for 32 bit input. I'd suggest
> > fixing the caller to use an unsigned type.
> 
> Then,
> Cc: Len Brown <len.brown@intel.com> (supporter:SUSPEND TO RAM)
> Cc: Pavel Machek <pavel@ucw.cz> (supporter:SUSPEND TO RAM)
> Cc: "Rafael J. Wysocki" <rjw@sisk.pl> (supporter:SUSPEND TO RAM)
> Cc: linux-pm@lists.linux-foundation.org (open list:SUSPEND TO RAM)

Yes, that sounds right.

	Arnd