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

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Date:   Fri, 31 Dec 2021 11:04:12 -0500
From:   Arnd Bergmann <arnd@...nel.org>
To:     Michael Schmitz <schmitzmic@...il.com>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Niklas Schnelle <schnelle@...ux.ibm.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
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Subject: Re: [RFC 02/32] Kconfig: introduce HAS_IOPORT option and select it as necessary

On Wed, Dec 29, 2021 at 10:44 PM Michael Schmitz <schmitzmic@...il.com> wrote:
> Am 30.12.2021 um 14:48 schrieb Arnd Bergmann:
> > On Tue, Dec 28, 2021 at 11:15 PM Michael Schmitz <schmitzmic@...il.com> wrote:
> > What some other architectures do is to rely on inb()/outb() to have a
> > zero-based offset, and use an io_offset in PCI buses to ensure that a
> > low port number on the bus gets translated into a pointer value for the
> > virtual mapping in the kernel, which is then represented as an unsigned
> > int.
>
> M54xx does just that for Coldfire:
>
> arch/m68k/include/asm/io_no.h:
> #define PCI_IO_PA       0xf8000000              /* Host physical address */
>
> (used to set PCI BAR mappings, so matches your definition above).

I think coldfire gets it right here, using PCI_IOBASE to find the start of
the window and a zero io_offset:

#define PCI_IOBASE ((void __iomem *) PCI_IO_PA)

> All other (MMU) m68k users of inb()/outb() apply an io_offset in the
> platform specific address translation:
>
> arch/m68k/include/asm/io_mm.h:
>
> #define q40_isa_io_base  0xff400000
> #define enec_isa_read_base  0xfffa0000
> #define enec_isa_write_base 0xfffb0000
>
> arch/m68k/include/asm/amigayle.h:
>
> #define GAYLE_IO                (0xa20000+zTwoBase)     /* 16bit and
> even 8bit registers */
> #define GAYLE_IO_8BITODD        (0xa30000+zTwoBase)     /* odd 8bit
> registers */
>
> (all constants used in address translation inlines that are used by the
> m68k inb()/outb() macros - you can call that the poor man's version of
> PCI BAR mappings ...).

This still looks like the same thing to me, where you have inb() take a
zero-based port number, not a pointer. The effect is the same as the
coldfire version, it just uses a custom inline function instead of the
version from asm-generic/io.h.

> So as long as support for any of the m68k PCI or ISA bridges is selected
> in the kernel config, the appropriate IO space mapping is applied. If no
> support for PCI or ISA bridges is selected, we already fall back to zero
> offset mapping (but as far as I can tell, it shouldn't be possible to
> build a kernel without bridge support but drivers that require it).

Right.

> > As this is indistinguishable from architectures that just don't have
> > a base address for I/O ports (we unfortunately picked 0 as the default
> > PCI_IOBASE value), my suggestion was to start marking architectures
> > that may have this problem as using HAS_IOPORT in order to keep
> > the existing behavior unchanged. If m68k does not suffer from this,
> > making HAS_IOPORT conditional on those config options that actually
> > need it would of course be best.
>
> Following your description, HAS_IOPORT would be required for neither of
> PCI, ISA or ATARI_ROM_ISA ??

For these three options, we definitely need HAS_IOPORT, which would
imply that some version of inb()/outb() is provided. The difference between
using a custom PCI_IOBASE (or an open-coded equivalent) and using
a zero PCI_IOBASE in combination with registering PCI using a custom
io_offset is whether we can use drivers with hardcoded port numbers.
These should depend on a different Kconfig symbol to be introduced
(CONFIG_HARDCODED_IOPORT or similar) once we introduce them,
and you could decide for m68k whether to allow those or not, I would
assume you do want them in order to use certain legacy ISA drivers.

       Arnd