linux-kernel - Re: [PATCH 3/3 v2] sh: fix READ/WRITE redefinition warnings

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Date:   Thu, 1 Jul 2021 13:57:13 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        Randy Dunlap <rdunlap@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        Linux-sh list <linux-sh@...r.kernel.org>,
        Takashi YOSHII <takasi-y@....dti.ne.jp>
Subject: Re: [PATCH 3/3 v2] sh: fix READ/WRITE redefinition warnings

On Thu, Jul 1, 2021 at 10:41 AM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> On Wed, Jun 30, 2021 at 9:36 PM John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de> wrote:
> > On 6/30/21 9:33 PM, Randy Dunlap wrote:
> > > On 6/30/21 2:36 AM, John Paul Adrian Glaubitz wrote:
>
> > FWIW, there are some warnings when building the SH-7785LCR configuration:
> >
> > In file included from ./arch/sh/include/asm/hw_irq.h:6,
> >                  from ./include/linux/irq.h:591,
> >                  from ./include/asm-generic/hardirq.h:17,
> >                  from ./arch/sh/include/asm/hardirq.h:9,
> >                  from ./include/linux/hardirq.h:11,
> >                  from ./include/linux/interrupt.h:11,
> >                  from ./include/linux/serial_core.h:13,
> >                  from ./include/linux/serial_sci.h:6,
> >                  from arch/sh/kernel/cpu/sh4a/setup-sh7785.c:10:
> > ./include/linux/sh_intc.h:100:63: warning: division 'sizeof (void *) / sizeof (void)' does not compute the number of array elements [-Wsizeof-pointer-div]
> >   100 | #define _INTC_ARRAY(a) a, __same_type(a, NULL) ? 0 : sizeof(a)/sizeof(*a)
> >       |                                                               ^
> > ./include/linux/sh_intc.h:107:9: note: in expansion of macro '_INTC_ARRAY'
> >   107 |         _INTC_ARRAY(sense_regs), _INTC_ARRAY(ack_regs), \
> >       |         ^~~~~~~~~~~
> > ./include/linux/sh_intc.h:124:15: note: in expansion of macro 'INTC_HW_DESC'
> >   124 |         .hw = INTC_HW_DESC(vectors, groups, mask_regs,                  \
> >       |               ^~~~~~~~~~~~
> > arch/sh/kernel/cpu/sh4a/setup-sh7785.c:478:8: note: in expansion of macro 'DECLARE_INTC_DESC'
> >   478 | static DECLARE_INTC_DESC(intc_desc, "sh7785", vectors, groups,
> >       |        ^~~~~~~~~~~~~~~~~
>
> A while ago, I had a look into fixing them, but it was non-trivial.
> The issue is that the macros are sometimes used with NULL pointer arrays.
> The __same_type() check in
>
>     #define _INTC_ARRAY(a) a, __same_type(a, NULL) ? 0 : sizeof(a)/sizeof(*a)
>
> is supposed to catch that, but modern compilers seem to evaluate the
> non-taken branch, too, leading to the warning.
>
> Anyone with a suggestion? (CCing the multi-compiler guru)

I think I looked at this (or maybe a related sh driver) in the past
and also gave up.

The best idea I have would be to manually expand the DECLARE_INTC_DESC()
macros. It would be possible to add further variations of them for each
combination of NULL/non-NULL arguments, but IMHO they don't actually
add any readability over the expanded version in the first place.

Another option would be to change the definition from array/size to
zero-terminated
arrays and get rid of the sizeof hack.

      Arnd