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

[Arnd Bergmann <arnd@arndb.de>]

On Tue, Apr 13, 2021 at 3:06 PM David Laight <David.Laight@aculab.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