linux-kernel - Re: [PATCH 4/4] lib/iomap.c: improve comment about pci anomaly

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Message-Id: <a9ab9976-c1e0-4f91-b17f-e5bbbf21def3@app.fastmail.com>
Date:   Tue, 21 Nov 2023 11:03:06 +0100
From:   "Arnd Bergmann" <arnd@...nel.org>
To:     "Philipp Stanner" <pstanner@...hat.com>,
        "Bjorn Helgaas" <bhelgaas@...gle.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Randy Dunlap" <rdunlap@...radead.org>,
        "Jason Gunthorpe" <jgg@...pe.ca>,
        "Eric Auger" <eric.auger@...hat.com>,
        "Kent Overstreet" <kent.overstreet@...il.com>,
        "Niklas Schnelle" <schnelle@...ux.ibm.com>,
        "Neil Brown" <neilb@...e.de>, "John Sanpe" <sanpeqf@...il.com>,
        "Dave Jiang" <dave.jiang@...el.com>,
        "Yury Norov" <yury.norov@...il.com>,
        "Kees Cook" <keescook@...omium.org>,
        "Masami Hiramatsu" <mhiramat@...nel.org>,
        "David Gow" <davidgow@...gle.com>,
        "Herbert Xu" <herbert@...dor.apana.org.au>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "wuqiang.matt" <wuqiang.matt@...edance.com>,
        "Jason Baron" <jbaron@...mai.com>,
        "Ben Dooks" <ben.dooks@...ethink.co.uk>,
        "Danilo Krummrich" <dakr@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH 4/4] lib/iomap.c: improve comment about pci anomaly

On Mon, Nov 20, 2023, at 22:59, Philipp Stanner wrote:
> lib/iomap.c contains one of the definitions of pci_iounmap(). The
> current comment above this out-of-place function does not clarify WHY
> the function is defined here.
>
> Linus's detailed comment above pci_iounmap() in drivers/pci/iomap.c
> clarifies that in a far better way.
>
> Extend the existing comment with an excerpt from Linus's and hint at the
> other implementation in drivers/pci/iomap.c
>
> Signed-off-by: Philipp Stanner <pstanner@...hat.com>

I think instead of explaining why the code is so complicated
here, I'd prefer to make it more logical and not have to
explain it.

We should be able to define a generic version like

void pci_iounmap(struct pci_dev *dev, void __iomem * addr)
{
#ifdef CONFIG_HAS_IOPORT
       if (iomem_is_ioport(addr)) {
              ioport_unmap(addr);
              return;
       }
#endif
      iounmap(addr)
}

and then define iomem_is_ioport() in lib/iomap.c for x86,
while defining it in asm-generic/io.h for the rest,
with an override in asm/io.h for those architectures
that need a custom inb().

Note that with ia64 gone, GENERIC_IOMAP is not at all
generic any more and could just move it to x86 or name
it something else. This is what currently uses it:

arch/hexagon/Kconfig:   select GENERIC_IOMAP
arch/um/Kconfig:        select GENERIC_IOMAP

These have no port I/O at all, so it doesn't do anything.

arch/m68k/Kconfig:      select GENERIC_IOMAP

on m68knommu, the default implementation from asm-generic/io.h
as the same effect as GENERIC_IOMAP but is more efficient.
On classic m68k, GENERIC_IOMAP does not do what it is
meant to because I/O ports on ISA devices have port
numbers above PIO_OFFSET. Also they don't have PCI.

arch/mips/Kconfig:      select GENERIC_IOMAP

This looks completely bogus because it sets PIO_RESERVED
to 0 and always uses the mmio part of lib/iomap.c. 

arch/powerpc/platforms/Kconfig: select GENERIC_IOMAP

This is only used for two platforms: cell and powernv,
though on Cell it no longer does anything after the
commit f4981a00636 ("powerpc: Remove the celleb support");
I think the entire io_workarounds code now be folded
back into spider_pci.c if we wanted to.

The PowerNV LPC support does seem to still rely on it.
This tries to do the exact same thing as lib/logic_pio.c
for Huawei arm64 servers. I suspect that neither of them
does it entirely correctly since the powerpc side appears
to just override any non-LPC PIO support while the arm64
side is missing the ioread/iowrite support.

     Arnd