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

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Date:   Fri, 06 May 2022 13:18:51 +0200
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     Finn Thain <fthain@...ux-m68k.org>,
        Bjorn Helgaas <helgaas@...nel.org>
Cc:     Arnd Bergmann <arnd@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Michal Simek <monstr@...str.eu>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "open list:ALPHA PORT" <linux-alpha@...r.kernel.org>,
        "moderated list:ARM PORT" <linux-arm-kernel@...ts.infradead.org>,
        "open list:IA64 (Itanium) PLATFORM" <linux-ia64@...r.kernel.org>,
        "open list:M68K ARCHITECTURE" <linux-m68k@...ts.linux-m68k.org>,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        "open list:PARISC ARCHITECTURE" <linux-parisc@...r.kernel.org>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>,
        "open list:RISC-V ARCHITECTURE" <linux-riscv@...ts.infradead.org>,
        "open list:SUPERH" <linux-sh@...r.kernel.org>,
        "open list:SPARC + UltraSPARC (sparc/sparc64)" 
        <sparclinux@...r.kernel.org>
Subject: Re: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select
 it as necessary

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@...nel.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.
> > 
> 
> 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@mail.gmail.com/