On Wed, Mar 28, 2018 at 8:56 AM, Benjamin Herrenschmidt <b...@kernel.crashing.org> wrote: > On Wed, 2018-03-28 at 06:53 +0000, Linus Torvalds wrote: >> On Tue, Mar 27, 2018, 20:43 Benjamin Herrenschmidt >> <b...@kernel.crashing.org> 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