From mboxrd@z Thu Jan  1 00:00:00 1970
From: Will Deacon <will.deacon@arm.com>
Subject: Re: RFC on writel and writel_relaxed
Date: Wed, 28 Mar 2018 09:53:39 +0100
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To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Arnd Bergmann <arnd@arndb.de>, Jonathan Corbet <corbet@lwn.net>, "linux-rdma@vger.kernel.org" <linux-rdma@vger.kernel.org>, Sinan Kaya <okaya@codeaurora.org>, Jason Gunthorpe <jgg@ziepe.ca>, Peter Zijlstra <peterz@infradead.org>, David Laight <David.Laight@aculab.com>, Oliver <oohall@gmail.com>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" <linuxppc-dev@lists.ozlabs.org>, Ingo Molnar <mingo@redhat.com>
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On Wed, Mar 28, 2018 at 08:29:45AM +1100, Benjamin Herrenschmidt wrote:
> On Tue, 2018-03-27 at 15:36 +0100, Will Deacon wrote:
> > > Can we say the same thing for iowrite32() and iowrite32be(). I also see wmb()
> > > in front of these.
> > 
> > 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.


For arm/arm64 these end up behaving exactly the same as readX/writeX, but
I'm nervous about changing the documentation without understanding why it's
like it is currently. Maybe another ia64 thing?.

Will