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Date:   Thu, 5 May 2022 14:53:42 -0500
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     Arnd Bergmann <arnd@kernel.org>
Cc:     Niklas Schnelle <schnelle@linux.ibm.com>,
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Subject: Re: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select
 it as necessary
Message-ID: <20220505195342.GA509942@bhelgaas>
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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