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

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Date:   Fri, 24 Nov 2023 20:08:27 +0100
From:   Danilo Krummrich <dakr@...hat.com>
To:     Arnd Bergmann <arnd@...nel.org>,
        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>
Cc:     linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH 4/4] lib/iomap.c: improve comment about pci anomaly

Hi Arnd,

On 11/21/23 11:03, 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.
>>
>> 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)

Let's shed some light on the different config options related to this.

To me it looks like GENERIC_IOMAP always implies GENERIC_PCI_IOMAP.

lib/iomap.c contains a definition of pci_iounmap() since it uses the
common IO_COND() macro. This definitions wins if GENERIC_IOMAP was
selected.

lib/pci_iomap.c contains another definition of pci_iounmap() which is
guarded by ARCH_WANTS_GENERIC_PCI_IOUNMAP to prevent multiple definitions
in case either GENERIC_IOMAP is set or the architecture already defined
pci_iounmap().

What's the purpose of not having set ARCH_HAS_GENERIC_IOPORT_MAP producing
an empty definition of pci_iounmap() though [1]?

And more generally, is there any other (subtle) logic behind this?

[1] https://elixir.bootlin.com/linux/latest/source/lib/pci_iomap.c#L167

> {
> #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().

So, that would be similar to IO_COND(), right? What would we need inb() for
in this context?

- Danilo

> 
> 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
>