On Wed, Mar 28, 2018 at 09:00:01AM +0000, David Laight wrote:

From: Will Deacon

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Sent: 28 March 2018 09:54

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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