On Fri, May 6, 2022 at 2:56 PM David Laight [off-list ref] wrote:

From: Maciej W. Rozycki

quoted

Sent: 06 May 2022 13:27
On Fri, 6 May 2022, Arnd Bergmann wrote:

quoted

quoted

 If this is PCI/PCIe indeed, then an I/O access is just a different bit
pattern put on the bus/in the TLP in the address phase.  So what is there
inherent to the s390 architecture that prevents that different bit pattern
from being used?

The hardware design for PCI on s390 is very different from any other
architecture, and more abstract. Rather than implementing MMIO register
access as pointer dereference, this is a separate CPU instruction that
takes a device/bar plus offset as arguments rather than a pointer, and
Linux encodes this back into a fake __iomem token.

 OK, that seems to me like a reasonable and quite a clean design (on the
hardware side).

 So what happens if the instruction is given an I/O rather than memory BAR
as the relevant argument?  Is the address space indicator bit (bit #0)
simply ignored or what?

You don't understand something...

For a memory BAR pci_ioremap() returns a token that can be used with readl().
On most architectures readl() is just a memory access.
This all fails on an I/O BAR.

For an I/O BAR you want a similar pair of functions.
On x86 the access function would need to use the 'in/out' instructions but
there is no requirement for the token to be the native io port number.
On many non-x86 architectures the access function would be a simple memory
access - but a specific system (eg many ppc) may never actually allow
such mappings to succeed.

You might also want a third PCI mapping function that can map a memory
BAR or an I/O BAR - with a suitable access function.
On x86 this would need something like ioread8() for accesses.
Except that function uses small integers for io port numbers
(which is inherently dangerous).

Nothing except the architecture specific implementation of the function
to access an io BAR would ever go near a function called inb().

The same is really true for other bus type - including ISA and EISA.
(Ignoring the horrid of probing ISI bus devices - hopefully they
are in the ACPI tables??_
If a driver is probed on a ISA bus there ought to be functions
equivalent to pci_ioremap() (for both memory and IO addresses)
that return tokens appropriate for the specific bus.

That is all a different load of churn.

A loooong time ago,  it was suggested to add register accessor
functions to struct device, so e.g. readl(dev, offset) would call
into these accessors, which would implement the bus-specific behavior.
No more worries about readl(), __raw_readl(), ioread32b(), or whatever
quirk is needed, at the (small on nowadays' machines) expense of
some indirection...

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds