linux-kernel - Re: [PATCH v2 4/4] s390: fix clang -Wpointer-sign warnigns in boot code

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Message-ID: <CAKwvOdmrOdzb3N3oNZL1kKYYSKzrVR+Opc7af9cYDL9cd2QpOg@mail.gmail.com>
Date:   Tue, 23 Apr 2019 11:21:28 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Collin Walling <walling@...ux.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/4] s390: fix clang -Wpointer-sign warnigns in boot code

On Tue, Apr 23, 2019 at 1:06 AM Arnd Bergmann <arnd@...db.de> wrote:
>
> On Mon, Apr 22, 2019 at 7:52 PM 'Nick Desaulniers' via Clang Built
> Linux <clang-built-linux@...glegroups.com> wrote:
>
> > > @@ -20,7 +20,7 @@ extern __u8 _ebc_tolower[256]; /* EBCDIC -> lowercase */
> > >  extern __u8 _ebc_toupper[256]; /* EBCDIC -> uppercase */
> > >
> > >  static inline void
> > > -codepage_convert(const __u8 *codepage, volatile __u8 * addr, unsigned long nr)
> > > +codepage_convert(const __u8 *codepage, volatile char* addr, unsigned long nr)
> > >  {
> > >         if (nr-- <= 0)
> > >                 return;
> >
> > There are many call sites of ASCEBC which is defined in terms of this
> > function. Do they all use `char*`?  grep shows an explicit cast to
> > `unsigned char*` in drivers/s390/char/tape_std.c for example.
>
> Generally speaking, the kernel is full of Wpointer-sign warnings, that's why
> this warning is disabled in the top-level Makefile by default. My patch fixes
> the ones in the s390 boot code that is not built with those default flags, but
> I made no attempt to fix the rest of the kernel.

Right, sorry, I forgot about that.  This patch looks good to me.
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

>
> Fun fact: on most architectures, 'char' is signed, but on s390 and 32-bit
> arm it is unsigned. The compiler treats 'char', 'unsigned char' and
> 'signed char'
> as three distinct types here for that reason.

I think I recall reading about that in:
https://www.amazon.com/ARM-System-Developers-Guide-Architecture/dp/1558608745
(I'll try to dig it up and post what the explanation was).

-- 
Thanks,
~Nick Desaulniers