Re: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select it as necessary

[Niklas Schnelle]

On Fri, 2022-05-06 at 19:12 +1000, Finn Thain wrote:
> 
> On Thu, 5 May 2022, Bjorn Helgaas wrote:
> 
> > 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@xxxxxxxxxx> 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.
> > 
> 
> Do you mean, "the most practical way to avoid a compiler warning on s390"? 
> What about "#pragma GCC diagnostic ignored"?

This actually happens with clang. Apart from that, I think this would
also fall under the same argument as the original patch Linus unpulled.
We would just paint over someting that we know at compile time won't
work:

https://lore.kernel.org/lkml/CAHk-=wg80je=K7madF4e7WrRNp37e3qh6y10Svhdc7O8SZ_-8g@xxxxxxxxxxxxxx/