On Wed, Mar 28, 2018 at 8:56 AM, Benjamin Herrenschmidt
[off-list ref] wrote:

On Wed, 2018-03-28 at 06:53 +0000, Linus Torvalds wrote:

quoted

On Tue, Mar 27, 2018, 20:43 Benjamin Herrenschmidt [off-list ref] wrote:
That's why in/out were *so* slow, and why nobody uses them any more
(well, the address size limitations and the lack of any remapping of
the address obviously also are a reason).

All true indeed, though a lot of other archs never quite made them
fully synchronous, which was another can of worms ... oh well.

Many architectures have no way of providing PCI compliant semantics
for outb, as their instruction set and/or bus interconnect lacks a
method of waiting for completion of an outb.

In practice, it doesn't seem to matter for any of the devices one would
encounter these days: very few use I/O space, and those that do don't
actually rely on the strict ordering. Some architectures (in particular
s390, but I remember seeing the same thing elsewhere) explicitly
disallow I/O space access on PCI because of this. On ARM, the typical
PCI implementations have other problems that are worse than this
one, so most drivers are fine with the almost-working semantics.

        Arnd