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

[Philipp Stanner]

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.
> > 
> > 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@xxxxxxxxxx>
> 
> 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)
> }

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

> 
> 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

> 
> 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.

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.

> 
>      Arnd
>