From mboxrd@z Thu Jan  1 00:00:00 1970
From: Bjorn Helgaas <helgaas@kernel.org>
Subject: Re: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select
 it as necessary
Date: Thu, 5 May 2022 14:53:42 -0500
Message-ID: <20220505195342.GA509942@bhelgaas>
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To: Arnd Bergmann <arnd@kernel.org>
Cc: Niklas Schnelle <schnelle@linux.ibm.com>, Arnd Bergmann <arnd@arndb.de>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-arch <linux-arch@vger.kernel.org>, linux-pci <linux-pci@vger.kernel.org>, Richard Henderson <rth@twiddle.net>, Ivan Kokshaysky <ink@jurassic.park.msu.ru>, Matt Turner <mattst88@gmail.com>, Russell King <linux@armlinux.org.uk>, Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>, Geert Uytterhoeven <geert@linux-m68k.org>, Michal Simek <monstr@monstr.eu>, Thomas Bogendoerfer <tsbogend@alpha.franken.de>, "James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>, Helge Deller <deller@gmx.de>, Michael Ellerman <mpe@ellerman.id.au>, Paul

On Thu, May 05, 2022 at 07:39:42PM +0200, Arnd Bergmann wrote:
> On Thu, May 5, 2022 at 6:10 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > On Wed, May 04, 2022 at 11:31:28PM +0200, Arnd Bergmann wrote:
> > >
> > > The main goal is to avoid c), which is what happens on s390, but
> > > can also happen elsewhere. Catching b) would be nice as well,
> > > but is much harder to do from generic code as you'd need an
> > > architecture specific inline asm statement to insert a ex_table
> > > fixup, or a runtime conditional on each access.
> >
> > Or s390 could implement its own inb().
> >
> > I'm hearing that generic powerpc kernels have to run both on machines
> > that have I/O port space and those that don't.  That makes me think
> > s390 could do something similar.
> 
> No, this is actually the current situation, and it makes absolutely no
> sense. s390 has no way of implementing inb()/outb() because there
> are no instructions for it and it cannot tunnel them through a virtual
> address mapping like on most of the other architectures. (it has special
> instructions for accessing memory space, which is not the same as
> a pointer dereference here).
> 
> The existing implementation gets flagged as a NULL pointer dereference
> by a compiler warning because it effectively is.

I think s390 currently uses the inb() in asm-generic/io.h, i.e.,
"__raw_readb(PCI_IOBASE + addr)".  I understand that's a NULL pointer
dereference because the default PCI_IOBASE is 0.

I mooted a s390 inb() implementation like "return ~0" because that's
what happens on most arches when there's no device to respond to the
inb().

The HAS_IOPORT dependencies are fairly ugly IMHO, and they clutter
drivers that use I/O ports in some cases but not others.  But maybe
it's the most practical way.

Bjorn