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

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Date:   Tue, 13 Apr 2021 14:12:03 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arnd Bergmann' <arnd@...db.de>
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

From: Arnd Bergmann
> Sent: 13 April 2021 14:40
> 
> 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.

It must have been the VMEbus (and maybe sBus) sparc that used an ASI.

I do remember issues with Solaris of some PCI cards not liking
being assigned a BAR address of zero.
That may be why the low 4k IO space isn't assigned here.
(I've never run Linux on sparc, just SVR4 and Solaris.)

I guess setting PCI_IOBASE to zero is safer when you can't trust
drivers not to use inb() instead of readb().
Or whatever io_read() ends up being.

	David

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