RFC on writel and writel_relaxed

[David Laight]

From: Will Deacon
> Sent: 28 March 2018 09:54
...
> > > I don't think so. My reading of memory-barriers.txt says that writeX might
> > > expand to outX, and outX is not ordered with respect to other types of
> > > memory.
> >
> > Ugh ?
> >
> > My understanding of HW at least is the exact opposite. outX is *more*
> > ordered if anything, than any other accessors. IO space is completely
> > synchronous, non posted and ordered afaik.
> 
> I'm just going by memory-barriers.txt:
> 
> 
>  (*) inX(), outX():
> 
>      [...]
> 
>      They are guaranteed to be fully ordered with respect to each other.
> 
>      They are not guaranteed to be fully ordered with respect to other types of
>      memory and I/O operation.

A long time ago there was a document from Intel that said that inb/outb weren't
necessarily synchronised wrt memory accesses.
(Might be P-pro era).
However no processors actually behaved that way and more recent docs
say that inb/outb are fully ordered.

	David