linux-kernel - RE: [PATCH v4] Kconfig: introduce HAS_IOPORT option and select it as necessary

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Date:   Wed, 5 Apr 2023 21:36:44 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arnd Bergmann' <arnd@...db.de>, "H. Peter Anvin" <hpa@...or.com>,
        "Niklas Schnelle" <schnelle@...ux.ibm.com>,
        Richard Henderson <richard.henderson@...aro.org>,
        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>,
        Huacai Chen <chenhuacai@...nel.org>,
        WANG Xuerui <kernel@...0n.name>,
        "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>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Yoshinori Sato <ysato@...rs.osdn.me>,
        Rich Felker <dalias@...c.org>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        "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" <x86@...nel.org>
CC:     "linux-m68k@...r.kernel.org" <linux-m68k@...r.kernel.org>,
        "linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        "linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        "loongarch@...ts.linux.dev" <loongarch@...ts.linux.dev>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Mauro Carvalho Chehab" <mchehab@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Arnd Bergmann <arnd@...nel.org>,
        "linux-parisc@...r.kernel.org" <linux-parisc@...r.kernel.org>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-alpha@...r.kernel.org" <linux-alpha@...r.kernel.org>,
        Johannes Berg <johannes@...solutions.net>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: RE: [PATCH v4] Kconfig: introduce HAS_IOPORT option and select it as
 necessary

From: Linuxppc-dev Arnd Bergmann
> Sent: 05 April 2023 21:32
> 
> On Wed, Apr 5, 2023, at 22:00, H. Peter Anvin wrote:
> > On April 5, 2023 8:12:38 AM PDT, Niklas Schnelle <schnelle@...ux.ibm.com> wrote:
> >>On Thu, 2023-03-23 at 17:33 +0100, Niklas Schnelle wrote:
> >>> We introduce a new HAS_IOPORT Kconfig option to indicate support for I/O
> >>> Port access. In a future patch HAS_IOPORT=n will disable compilation of
> >>> the I/O accessor functions inb()/outb() and friends on architectures
> >>> which can not meaningfully support legacy I/O spaces such as s390.
> >>> >>
> >>Gentle ping. As far as I can tell this hasn't been picked to any tree
> >>sp far but also hasn't seen complains so I'm wondering if I should send
> >>a new version of the combined series of this patch plus the added
> >>HAS_IOPORT dependencies per subsystem or wait until this is picked up.
> >
> > You need this on a system supporting not just ISA but also PCI.
> >
> > Typically on non-x86 architectures this is simply mapped into a memory window.
> 
> I'm pretty confident that the list is correct here, as the HAS_IOPORT
> symbol is enabled exactly for the architectures that have a way to
> map the I/O space. PCIe generally works fine without I/O space, the
> only exception are drivers for devices that were around as early PCI.

Isn't there a difference between cpu that have inb()/outb() (probably
only x86?) and architectures (well computer designs) that can generate
PCI 'I/O' cycles by some means.
It isn't even just PCI I/O cycles, I've used an ARM cpu (SA1100)
that mapped a chuck of physical address space onto PCMCIA I/O cycles.

If the hardware can map a PCI 'IO' bar into normal kernel address
space then the bar and accesses can be treated exactly like a memory bar.
This probably leaves x86 as the outlier where you need (IIRC) io_readl()
and friends that can generate in/out instructions for those accesses.

There are also all the x86 ISA devices which need in/out instructions.
But (with the likely exception of the UART) they are pretty much
platform specific.

So, to my mind at least, HAS_IOPORT is just the wrong question.

	David

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