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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Wed, 8 Sep 2021 11:50:12 +0200
Message-ID: <CAK8P3a3yJHvJaFHUh2+5GPm2n_g9gSfX2rFbrSLzDt6yC4eDog@mail.gmail.com>
Subject: Re: [PATCH] Enable '-Werror' by default for all kernel builds
To:     Geert Uytterhoeven <geert@linux-m68k.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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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.

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?
I see two other drivers accessing the nvram here

arch/m68k/mvme16x/config.c:static MK48T08ptr_t volatile rtc =
(MK48T08ptr_t)MVME_RTC_BASE;
arch/m68k/mvme16x/rtc.c:        volatile MK48T08ptr_t rtc =
(MK48T08ptr_t)MVME_RTC_BASE;

The same trick should work here, just create a local variable with a
volatile pointer and read from that.

       Arnd