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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Tue, 4 May 2021 09:42:42 +0200
Message-ID: <CAK8P3a1F1X2SUZLLxK8_QriRZjYyV2AYJFNQt=sGZcWP8yovaw@mail.gmail.com>
Subject: Re: [PATCH v4 0/3] asm-generic/io.h: Silence -Wnull-pointer-arithmetic
 warning on PCI_IOBASE
To:     Niklas Schnelle <schnelle@linux.ibm.com>
Cc:     Vineet Gupta <vgupta@synopsys.com>,
        "David S. Miller" <davem@davemloft.net>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-s390 <linux-s390@vger.kernel.org>,
        "open list:SYNOPSYS ARC ARCHITECTURE" 
        <linux-snps-arc@lists.infradead.org>,
        sparclinux <sparclinux@vger.kernel.org>
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On Tue, May 4, 2021 at 9:40 AM Niklas Schnelle <schnelle@linux.ibm.com> wrote:
> On Mon, 2021-05-03 at 18:07 +0200, Arnd Bergmann wrote:
> > - for the risc-v patch, I would suggest explaining that this fixes
> >   an existing runtime bug, not just a compiler error:
> >   | This is already broken, as accessing a fixed I/O port number of
> >   | an ISA device on NOMMU RISC-V would turn into a NULL pointer
> >   | dereference.
> >   Feel free to either copy this, or use your own explanation.
>
> I mixed the above in with the current commit message:
>
>     Without MMU support PCI_IOBASE is left undefined because PCI_IO_END is
>     VMEMMAP_START. Nevertheless the in*()/out*() helper macros are left
>     defined with uses of PCI_IOBASE. At the moment this only compiles
>     because asm-generic/io.h defines PCI_IOBASE as 0 if it is undefined and
>     so at macro expansion PCI_IOBASE is defined. This leads to compilation
>     errors when asm-generic/io.h is changed to leave PCI_IOBASE undefined.
>     More importantly it is currently broken at runtime, as accessing a fixed
>     I/O port number of an ISA device on NOMMU RISC-V would turn into a NULL
>     pointer dereference. Instead only define the in*()/out*() helper macros
>     with MMU support and fall back to the asm-generic/io.h helper stubs
>     otherwise.

Looks good, thanks. Maybe split into two or three paragraphs for readability.

     Arnd