linux-kernel - Re: [PATCH] asm-generic/io.h: Silence -Wnull-pointer-arithmetic warning on PCI

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Message-ID: <CAK8P3a3PK9zyeP4ymELtc2ZYnymECoACiigw9Za+pvSJpCk5=g@mail.gmail.com>
Date:   Tue, 13 Apr 2021 15:40:22 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     David Laight <David.Laight@...lab.com>
Cc:     Niklas Schnelle <schnelle@...ux.ibm.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Guo Ren <guoren@...nel.org>
Subject: Re: [PATCH] asm-generic/io.h: Silence -Wnull-pointer-arithmetic
 warning on PCI_IOBASE

On Tue, Apr 13, 2021 at 3:06 PM David Laight <David.Laight@...lab.com> wrote:
>
> From: Arnd Bergmann
> > Sent: 13 April 2021 13:58
> ...
> > The remaining ones (csky, m68k, sparc32) need to be inspected
> > manually to see if they currently support PCI I/O space but in
> > fact use address zero as the base (with large resources) or they
> > should also turn the operations into a NOP.
>
> I'd expect sparc32 to use an ASI to access PCI IO space.
> I can't quite remember whether IO space was supported at all.

I see this bit in arch/sparc/kernel/leon_pci.c

 * PCI Memory and Prefetchable Memory is direct-mapped. However I/O Space is
 * accessed through a Window which is translated to low 64KB in PCI space, the
 * first 4KB is not used so 60KB is available.
...
        pci_add_resource_offset(&resources, &info->io_space,
                                info->io_space.start - 0x1000);

which means that there is I/O space, which gets accessed through whichever
method readb() uses. Having the offset equal to the resource means that
the '(void *)0' start is correct.

As this leaves only two others, I checked those as well:

csky does not actually have a PCI host bridge driver at the moment, so
we don't care about breaking port access on it it, and I would suggest
leaving I/O port access disabled. (Added Guo Ren to Cc for confirmation).

m68k only supports PCI on coldfire M54xx, and this variant does set
a PCI_IOBASE after all. The normal MMU based m68k have no PCI
and do define their out inb/outb/..., so nothing changes for them.

To summarize: only sparc32 needs to set PCI_IOBASE to zero, everyone
else should just WARN_ONCE() or return 0xff/0xffff/0xffffffff.

        Arnd