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Date: Wed, 28 Mar 2018 10:09:07 +0100
From: Will Deacon <will.deacon@arm.com>
To: David Laight <David.Laight@ACULAB.COM>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
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Subject: Re: RFC on writel and writel_relaxed
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On Wed, Mar 28, 2018 at 09:00:01AM +0000, David Laight wrote:
> 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.

Thank you, David! I'll write another patch fixing this up and hopefully
we'll soon have one making writeX/readX much clearer.

Will