From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Laight <David.Laight@ACULAB.COM>
Date: Fri, 06 May 2022 13:28:42 +0000
Subject: RE: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select it as necessary
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To: "'Maciej W. Rozycki'" <macro@orcam.me.uk>
Cc: Arnd Bergmann <arnd@kernel.org>, Rich Felker <dalias@libc.org>, "open list:IA64 (Itanium) PLATFORM" <linux-ia64@vger.kernel.org>, "open list:SUPERH" <linux-sh@vger.kernel.org>, Catalin Marinas <catalin.marinas@arm.com>, Dave Hansen <dave.hansen@linux.intel.com>, "open list:MIPS" <linux-mips@vger.kernel.org>, "James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>, "open list:SPARC + UltraSPARC (sparc/sparc64)" <sparclinux@vger.kernel.org>, "open list:RISC-V ARCHITECTURE" <linux-riscv@lists.infradead.org>, Will Deacon <will@kernel.org>, linux-arch <linux-arch@vger.kernel.org>, Yoshinori Sato <ysato@users.sourceforge.jp>, Helge Deller <deller@gmx.de>, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>, Russell King <linux@armlinux.org.uk>, Ingo Molnar <mingo@redhat.com>, Geert Uytterhoeven <geert@linux-m68k.org>, linux-pci <linux-pci@vger.kernel.org>, Bjorn Helgaas <helgaas@kernel.org>, Matt Turner <mattst88@gmail.com>, Albert Ou <aou@eecs.berkeley.edu>, Arnd Bergmann <arnd@arndb.de>, Niklas Schnelle <schnelle@linux.ibm.com>, "open list:M68K ARCHITECTURE" <linux-m68k@lists.linux-m68k.org>, Ivan Kokshaysky <ink@jurassic.park.msu.ru>, Paul Walmsley <paul.walmsley@sifive.com>, Thomas Gleixner <tglx@linutronix.de>, "moderated list:ARM PORT" <linux-arm-kernel@lists.infradead.org>, Richard Henderson <rth@twiddle.net>, Michal Simek <monstr@monstr.eu>, Thomas Bogendoerfer <tsbogend@alpha.franken.de>, "open list:PARISC ARCHITECTURE" <linux-parisc@vger.kernel.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Palmer Dabbelt <palmer@dabbelt.com>, "open list:ALPHA PORT" <linux-alpha@vger.kernel.org>, Borislav Petkov <bp@alien8.de>, "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" <linuxppc-dev@lists.ozlabs.org>, "David S. Miller" <davem@davemloft.net>

From: Maciej W. Rozycki
> Sent: 06 May 2022 14:15
> On Fri, 6 May 2022, David Laight wrote:
> 
> > >  The PCI configuration space was retrofitted into x86 systems (and is
> > > accessed in an awkward manner with them), but with a new design such a
> > > clean approach is most welcome IMHO.  Thank you for your explanation.
> >
> > Actually I think x86 was the initial system for PCI.
> > The PCI config space 'mess' is all about trying to make
> > something that wouldn't break existing memory maps.
> 
>  It was retrofitted in that x86 systems already existed for ~15 years when
> PCI came into picture.  Therefore the makers of the CPU ISA couldn't have
> envisaged the need for config access instructions like they did for memory
> and port access.

Rev 2.0 of the PCI spec (1993) defines two mechanisms for config cycles.
#2 is probably the first one and maps all of PCI config space into
4k of IO space (PCI bridges aren't supported).
#1 requires a pair of accesses (and SMP locking).

Neither is really horrid.

For horrid try the ISApnp interface.

	David

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