linux-kernel - Re: [RFC 02/32] Kconfig: introduce HAS_IOPORT option and select it as necessary

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Date:   Fri, 31 Dec 2021 12:28:23 +0100
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     Michael Schmitz <schmitzmic@...il.com>,
        Arnd Bergmann <arnd@...nel.org>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        John Garry <john.garry@...wei.com>,
        Nick Hu <nickhu@...estech.com>,
        Greentime Hu <green.hu@...il.com>,
        Vincent Chen <deanbo422@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>, Guo Ren <guoren@...nel.org>,
        Dinh Nguyen <dinguyen@...nel.org>,
        Chris Zankel <chris@...kel.net>,
        Karol Gugala <kgugala@...micro.com>,
        Jeff Dike <jdike@...toit.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Brian Cain <bcain@...eaurora.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Vineet Gupta <vgupta@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.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>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        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>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-riscv@...ts.infradead.org, linux-csky@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, openrisc@...ts.librecores.org,
        linux-s390@...r.kernel.org, linux-alpha@...r.kernel.org,
        linux-snps-arc@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        Greg Ungerer <gerg@...ux-m68k.org>
Subject: Re: [RFC 02/32] Kconfig: introduce HAS_IOPORT option and select it
 as necessary

On Thu, 2021-12-30 at 16:44 +1300, Michael Schmitz wrote:
> Hi Arnd,
> 
> Am 30.12.2021 um 14:48 schrieb Arnd Bergmann:
> > On Tue, Dec 28, 2021 at 11:15 PM Michael Schmitz <schmitzmic@...il.com> wrote:
> > > Am 29.12.2021 um 16:41 schrieb Arnd Bergmann:
> > > > On Tue, Dec 28, 2021 at 8:20 PM Michael Schmitz <schmitzmic@...il.com> wrote:
---8<---
> 
> > What some other architectures do is to rely on inb()/outb() to have a
> > zero-based offset, and use an io_offset in PCI buses to ensure that a
> > low port number on the bus gets translated into a pointer value for the
> > virtual mapping in the kernel, which is then represented as an unsigned
> > int.
> 
> M54xx does just that for Coldfire:
> 
> arch/m68k/include/asm/io_no.h:
> #define PCI_IO_PA	0xf8000000		/* Host physical address */
> 
> (used to set PCI BAR mappings, so matches your definition above).
> 
> All other (MMU) m68k users of inb()/outb() apply an io_offset in the 
> platform specific address translation:
> 
> 
---8<---
> So as long as support for any of the m68k PCI or ISA bridges is selected 
> in the kernel config, the appropriate IO space mapping is applied. If no 
> support for PCI or ISA bridges is selected, we already fall back to zero 
> offset mapping (but as far as I can tell, it shouldn't be possible to 
> build a kernel without bridge support but drivers that require it).
> 
> > As this is indistinguishable from architectures that just don't have
> > a base address for I/O ports (we unfortunately picked 0 as the default
> > PCI_IOBASE value), my suggestion was to start marking architectures
> > that may have this problem as using HAS_IOPORT in order to keep
> > the existing behavior unchanged. If m68k does not suffer from this,
> > making HAS_IOPORT conditional on those config options that actually
> > need it would of course be best.
> 
> Following your description, HAS_IOPORT would be required for neither of 
> PCI, ISA or ATARI_ROM_ISA ??
> 

No, HAS_IOPORT being set just means that inb() etc. exist and are
functional be it as special instructions like on x86 or via an I/O
address offset. As I understand it if you do have PCI, ISA or
ATARI_ROM_ISA they are functional. If none of them are set and your
zero offset mapping means these accessors can't actually be used you
could make the declerations ifdeffed on CONFIG_HAS_IOPORT to detect the
cases where somone managed to build drivers that require them and that
would result in a compile time error instead of silently, or with a
NULL pointer warning, compiling code that won't work.