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

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Date:   Wed, 29 Dec 2021 17:15:23 +1300
From:   Michael Schmitz <schmitzmic@...il.com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Niklas Schnelle <schnelle@...ux.ibm.com>,
        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

Hi Arnd,

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:
>> Am 28.12.2021 um 23:08 schrieb Geert Uytterhoeven:
>>> On Mon, Dec 27, 2021 at 5:44 PM Niklas Schnelle <schnelle@...ux.ibm.com> wrote:
>>>> We introduce a new HAS_IOPORT Kconfig option to gate 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. On these platforms
>>>> inb()/outb() etc are currently just stubs in asm-generic/io.h which when
>>>> called will cause a NULL pointer access which some compilers actually
>>>> detect and warn about.
>>>>
>>>> The dependencies on HAS_IOPORT in drivers as well as ifdefs for
>>>> HAS_IOPORT specific sections will be added in subsequent patches on
>>>> a per subsystem basis. Then a final patch will ifdef the I/O access
>>>> functions on HAS_IOPORT thus turning any use not gated by HAS_IOPORT
>>>> into a compile-time warning.
>>>>
>>>> Link: https://lore.kernel.org/lkml/CAHk-=wg80je=K7madF4e7WrRNp37e3qh6y10Svhdc7O8SZ_-8g@mail.gmail.com/
>>>> Co-developed-by: Arnd Bergmann <arnd@...nel.org>
>>>> Signed-off-by: Arnd Bergmann <arnd@...nel.org>
>>>> Signed-off-by: Niklas Schnelle <schnelle@...ux.ibm.com>
>>>
>>> Thanks for your patch!
>>>
>>>> --- a/arch/m68k/Kconfig
>>>> +++ b/arch/m68k/Kconfig
>>>> @@ -16,6 +16,7 @@ config M68K
>>>>         select GENERIC_CPU_DEVICES
>>>>         select GENERIC_IOMAP
>>>>         select GENERIC_IRQ_SHOW
>>>> +       select HAS_IOPORT
>>>>         select HAVE_AOUT if MMU
>>>>         select HAVE_ASM_MODVERSIONS
>>>>         select HAVE_DEBUG_BUGVERBOSE
>>>
>>> This looks way too broad to me: most m68k platform do not have I/O
>>> port access support.
>>>
>>> My gut feeling says:
>>>
>>>     select HAS_IOPORT if PCI || ISA
>>>
>>> but that might miss some intricate details...
>>
>> In particular, this misses the Atari ROM port ISA adapter case -
>>
>>         select HAS_IOPORT if PCI || ISA || ATARI_ROM_ISA
>>
>> might do instead.
>
> Right, makes sense. I had suggested to go the easy way and assume that
> each architecture would select HAS_IOPORT if any configuration supports
> it, but it looks like for m68k there is a clearly defined set of platforms that
> do.
>
> Note that for the platforms that don't set any of the three symbols, the
> fallback makes inb() an alias for readb() with a different argument type,
> so there may be m68k specific drivers that rely on this, but those would
> already be broken if ATARI_ROM_ISA is set.

I'd hope not - we spent some effort to make sure setting ATARI_ROM_ISA 
does not affect other m68k platforms when e.g. building multiplatform 
kernels.

Replacing inb() by readb() without any address translation won't do much 
good for m68k though - addresses in the traditional ISA I/O port range 
would hit the (unmapped) zero page.

Cheers,

	Michael

>
>           Arnd
>