On Wed, 2018-03-28 at 09:11 +0200, Arnd Bergmann wrote: > 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.
Yup, that includes powerpc. Note that since POWER8 we don't even genetate IO space anymore :-) > 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. /me cries... Ben.