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Subject: Re: [RFC v2 02/39] ACPI: add dependency on HAS_IOPORT
From:   Niklas Schnelle <schnelle@linux.ibm.com>
To:     Arnd Bergmann <arnd@kernel.org>, Bjorn Helgaas <helgaas@kernel.org>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        linux-pci <linux-pci@vger.kernel.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        "open list:ACPI" <linux-acpi@vger.kernel.org>
Date:   Thu, 05 May 2022 10:20:28 +0200
In-Reply-To: <CAK8P3a3AddBGnBV=6wK+LZDjZD05k=9tBBWd7LWm6smXLcfREQ@mail.gmail.com>
References: <20220429135108.2781579-3-schnelle@linux.ibm.com>
         <20220504175352.GA456913@bhelgaas>
         <CAK8P3a3AddBGnBV=6wK+LZDjZD05k=9tBBWd7LWm6smXLcfREQ@mail.gmail.com>
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On Wed, 2022-05-04 at 21:58 +0200, Arnd Bergmann wrote:
> On Wed, May 4, 2022 at 7:53 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > On Fri, Apr 29, 2022 at 03:50:00PM +0200, Niklas Schnelle wrote:
> > > In a future patch HAS_IOPORT=n will result in inb()/outb() and friends
> > > not being declared. As ACPI always uses I/O port access we simply depend
> > > on HAS_IOPORT.
> > 
> > CONFIG_ACPI depends on ARCH_SUPPORTS_ACPI, which is only set by arm64,
> > ia64, and x86, all of which support I/O port access.  So does this
> > actually solve a problem?  I wouldn't think you'd be able to build
> > ACPI on s390 even without this patch.
> > "ACPI always uses I/O port access" is a pretty broad brush, and it
> > would be useful to know specifically what the dependencies are.
> > 
> > Many ACPI hardware accesses use acpi_hw_read()/acpi_hw_write(), which
> > use either MMIO or I/O port accesses depending on what the firmware
> > told us.
> 
> I think this came from my original prototype of the series where I tested it
> out on arm64 with HAS_IOPORT disabled. I would like to hide the definition
> of inb()/outb() from include/asm-generic/io.h whenever CONFIG_HAS_IOPORT
> is not set, and I was prototyping this on arm64.
> 
> There are uses of inb()/outb() in drivers/acpi/ec.c and drivers/acpi/osl.c,
> which in turn are not optional in ACPI, so it seems that those are
> required.
> 
> If we want to allow building arm64 without HAS_IOPORT for some reason,
> that means either force-disabling ACPI as well, or changin ACPI to not
> rely on port I/O. I think it's fine to leave that as a problem for whoever
> wants to make HAS_IOPORT optional in the future, and drop the
> dependency here.
> 
>        Arnd

I'll improve the commit message to make the dependency on HAS_IOPORT
more clear. I also agree with Arnd that since all architectures where
ACPI is useful have I/O ports making it work without I/O port access
compiled in is for another day.