linux-kernel - Re: [GIT PULL 1/2] asm-generic: rework PCI I/O space access

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Date:   Tue, 3 Aug 2021 14:15:21 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     John Garry <john.garry@...wei.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Niklas Schnelle <schnelle@...ux.ibm.com>
Subject: Re: [GIT PULL 1/2] asm-generic: rework PCI I/O space access

On Tue, Aug 3, 2021 at 1:23 PM John Garry <john.garry@...wei.com> wrote:
> > so I'm just not building those drivers any more, and not
> > defining the inb()/outb() helpers either, causing a build failure when I'm
> > missing an option.
> >
> > However it sounds like you are interested in a third option here, which
> > brings us to:
> >
> > LEGACY_PCI: any PCI driver that uses inb()/outb() or is only available
> >      on old-style PCI but not PCIe hardware without a bridge.
> >      To be disabled for most architectures and possibly distros but can
> >      be enabled for kernels that want to use those devices, as long as
> >      CONFIG_HAS_IOPORT is set by the architecture.
> >
> > HAS_IOPORT: not a legacy PCI device, but can only be built on
> >      architectures that define inb()/outb(). To be disabled for s390
> >      and any other machine that has no useful definition of those
> >      functions.
>
> That seems reasonable. And asm-generic io.h should be ifdef'ed by
> HAS_IOPORT. In your patch you had it under CONFIG_IOPORT - was that
> intentional?

No, that was a typo. Thanks for pointing this out.

> On another point, I noticed SCSI driver AHA152x depends on ISA, but is
> not an isa driver - however it does use port IO. Would such dependencies
> need to be changed to depend on HAS_IOPORT?

I'm not sure what you mean here. As far as I can tell, AHA152x is an ISA
driver in the sense that it is a driver for ISA add-on cards. However, it
is not a 'struct isa_driver' in the sense that AHA1542 is, AHA152x  is even
older and uses the linux-2.4 style initialization using a module_init()
function that does the probing.

> I did notice that arm32 support CONFIG_ISA - not sure why.

This is for some of the earlier machines we support:
mach-footbridge has some on-board ISA components, while
SA1100, PXA25x and S3C2410 each have at least one machine
with a PC/104 connector using ISA signaling for add-on cards.

There are also a couple of platforms with PCMCIA or CF slots
using the same ISA style I/O signals, but those have separate
drivers.

> > HARDCODED_IOPORT: (or another name you might think of,) Used by
> >     drivers that unconditionally do inb()/outb() without checking the
> >     validity of the address using firmware or other methods first.
> >     depends on HAS_IOPORT and possibly architecture specific
> >     settings.
>
> Yeah, that sounds the same as what I was thinking. Maybe IOPORT_NATIVE
> could work as a name. I would think that only x86/ia64 would define it.
> A concern though is that someone could argue that is a functional
> dependency, rather than just a build dependency.

You can have those on a number of platforms, such as early
PowerPC CHRP or pSeries systems, a number of MIPS workstations
including recent Loongson machines, and many Alpha platforms.

Maybe the name should reflect that these all use PC-style ISA/LPC
port numbers without the ISA connectors.

       Arnd