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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Wed, 8 Sep 2021 12:10:57 +0200
Message-ID: <CAMuHMdV51YuBYg6bvoQW9fBXFv8BRtnG9YTZST3AnbFwOsdW9g@mail.gmail.com>
Subject: Re: [PATCH] Enable '-Werror' by default for all kernel builds
To:     Arnd Bergmann <arnd@kernel.org>
Cc:     Guenter Roeck <linux@roeck-us.net>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "David S. Miller" <davem@davemloft.net>,
        Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        Christian Koenig <christian.koenig@amd.com>,
        Huang Rui <ray.huang@amd.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-sparc <sparclinux@vger.kernel.org>,
        Martin Sebor <msebor@gcc.gnu.org>
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Hi Arnd,

On Wed, Sep 8, 2021 at 11:50 AM Arnd Bergmann <arnd@kernel.org> wrote:
> On Wed, Sep 8, 2021 at 9:49 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > On Wed, Sep 8, 2021 at 7:16 AM Guenter Roeck <linux@roeck-us.net> wrote:
> > > On 9/7/21 9:48 PM, Al Viro wrote:
> > > > On Tue, Sep 07, 2021 at 09:28:38PM -0700, Guenter Roeck wrote:
> > > >>      memcpy(eth_addr, sanitize_address((void *) 0xfffc1f2c), ETH_ALEN);
> > > >>
> > > >> but that just seems weird. Is there a better solution ?
> > > >
> > > > (char (*)[ETH_ALEN])?  Said that, shouldn't that be doing something like
> > > > ioremap(), rather than casting explicit constants?
> > >
> > > Typecasts or even assigning the address to a variable does not help.
> > > The sanitizer function can not be static either.
> >
> > So it can only be fixed by obfuscating the constant address in a
> > chain of out-of-line functions...
> > How is this compiler to be used for bare-metal programming?
>
> I reported this as a gcc bug when I first saw it back in March:
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99578
>
> Martin Sebor suggested marking the pointer as 'volatile' as a workaround,
> which is probably fine for bare-metal programming, but I would consider
> that bad style for the kernel boot arguments. The RELOC_HIDE trick is probably
> fine here, as there are only a couple of instances, and for the network
> driver, using volatile is probably appropriate as well.

Yeah, volatile should be fine for drivers.
In fact this is one of the few places where I/O registers are accessed
without involving volatile.

> I still hope this can be fixed in a future gcc-11.x release. Maybe we should
> add further instances of the problem on the gcc bug to boost the priority?
>
> > > I don't know the hardware, so I can not answer the ioremap() question.
> >
> > Yes it should.  But this driver dates back to 2.1.110, when only
> > half of the architectures already had ioremap().
>
> How does  mvme16x even create the mapping? Is this a virtual address
> that is hardwired to the bus or do you have a static mapping somewhere?

It's part of the transparent mapping of the top address space for
I/O devices in arch/m68k/kernel/head.S.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds