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Message-ID: <10ef8eac12c01f52ec3b8c0bedf6cf34627d1ceb.camel@redhat.com>
Subject: Re: [PATCH 4/4] lib/iomap.c: improve comment about pci anomaly
From:   Philipp Stanner <pstanner@redhat.com>
To:     Arnd Bergmann <arnd@kernel.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Eric Auger <eric.auger@redhat.com>,
        Kent Overstreet <kent.overstreet@gmail.com>,
        Niklas Schnelle <schnelle@linux.ibm.com>,
        Neil Brown <neilb@suse.de>, John Sanpe <sanpeqf@gmail.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Yury Norov <yury.norov@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        David Gow <davidgow@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "wuqiang.matt" <wuqiang.matt@bytedance.com>,
        Jason Baron <jbaron@akamai.com>,
        Ben Dooks <ben.dooks@codethink.co.uk>,
        Danilo Krummrich <dakr@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org,
        pstanner@redhat.com
Date:   Wed, 29 Nov 2023 13:40:19 +0100
In-Reply-To: <a9ab9976-c1e0-4f91-b17f-e5bbbf21def3@app.fastmail.com>
References: <20231120215945.52027-2-pstanner@redhat.com>
         <20231120215945.52027-6-pstanner@redhat.com>
         <a9ab9976-c1e0-4f91-b17f-e5bbbf21def3@app.fastmail.com>
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Hi again,

so I thought about this for a while and want to ask some follow up
questions in addition to those by Danilo in the other mail.

(btw, -CC Herbert Xu, since the mailserver is bouncing)


On Tue, 2023-11-21 at 11:03 +0100, Arnd Bergmann wrote:
> 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.
> >=20
> > Linus's detailed comment above pci_iounmap() in drivers/pci/iomap.c
> > clarifies that in a far better way.
> >=20
> > Extend the existing comment with an excerpt from Linus's and hint
> > at the
> > other implementation in drivers/pci/iomap.c
> >=20
> > Signed-off-by: Philipp Stanner <pstanner@redhat.com>
>=20
> 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.
>=20
> We should be able to define a generic version like
>=20
> void pci_iounmap(struct pci_dev *dev, void __iomem * addr)
> {
> #ifdef CONFIG_HAS_IOPORT
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (iomem_is_ioport(addr)) {
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0 ioport_unmap(addr);
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0 return;
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 }
> #endif
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 iounmap(addr)
> }

ACK, I think this makes sense =E2=80=93 if we agree (as in the other thread=
)
that we never need an empty pci_iounmap().

>=20
> and then define iomem_is_ioport() in lib/iomap.c for x86,

Wait a minute.
lib/ should never contain architecture-specific code, should it? I
assume your argument is that we write a default version of
iomem_is_ioport(), that could, in theory, be used by many
architectures, but ultimately only x86 will use that default.

> while defining it in asm-generic/io.h for the rest,

So we're not talking about the function prototypes here, but about the
actual implementation that should reside in asm-generic/io.h, aye?
Because the prototype obviously always has to be identical.

> with an override in asm/io.h for those architectures
> that need a custom inb().

So like this in ARCH/include/asm/io.h:

#define iomem_is_ioport iomem_is_ioport
bool iomem_is_ioport(...);

and in include/asm-generic/io.h:

#ifndef iomem_is_ioport
bool iomem_is_ioport(...);

correct?

Still, as Danilo has asked in his email, the question about how inb()
is related to it still stands

>=20
> 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:
>=20
> arch/hexagon/Kconfig:=C2=A0=C2=A0 select GENERIC_IOMAP
> arch/um/Kconfig:=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 select GENERIC=
_IOMAP
>=20
> These have no port I/O at all, so it doesn't do anything.
>=20
> arch/m68k/Kconfig:=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 select GENERIC_IOMAP
>=20
> 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.
>=20
> arch/mips/Kconfig:=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 select GENERIC_IOMAP
>=20
> This looks completely bogus because it sets PIO_RESERVED
> to 0 and always uses the mmio part of lib/iomap.c.=20
>=20
> arch/powerpc/platforms/Kconfig: select GENERIC_IOMAP
>=20
> 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.
>=20
> 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.

I think by now I get what the issue with GENERIC_IOMAP is. But do you
want me to do something about GENERIC_IOMAP (besides the things
directly related to the PCI functionality I'm touching) for you to
approve of a modified version of this patch series?


P.

>=20
> =C2=A0=C2=A0=C2=A0=C2=A0 Arnd
>=20