linux-kernel - Re: [PATCH] docs/memory-barriers.txt: Fix broken DMA vs MMIO ordering example

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Date:   Wed, 28 Mar 2018 20:03:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tony Luck <tony.luck@...el.com>
Cc:     Sinan Kaya <okaya@...eaurora.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Will Deacon <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-doc@...r.kernel.org,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Arnd Bergmann <arnd@...db.de>, Jason Gunthorpe <jgg@...pe.ca>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        "linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>
Subject: Re: [PATCH] docs/memory-barriers.txt: Fix broken DMA vs MMIO
 ordering example

On Wed, Mar 28, 2018 at 10:57:11AM -0700, Tony Luck wrote:
> On Wed, Mar 28, 2018 at 6:02 AM, Sinan Kaya <okaya@...eaurora.org> wrote:
> > +linux-ia64
> > Does IA64 follow this requirement? If not, is implementation planned?
> >
> > "no wmb() before writel()"
> >
> > Linus asked us to get rid of wmb() in front of writel() for UC memory.
> > Just checking that we are not breaking anything for IA64.
> 
> We should be OK on ia64, writel() uses a cast to:
> 
>  *(volatile unsigned int __force *)
> 
> which the compiler takes as a request to use a "st4.rel" instruction
> (meaning "store with release semantics"). So the value stored will
> be visible to anything that follows.

Just to nitpick, regular release semantics don't guarantee anything like
that, but ia64 never actually got around to implementing proper release
and it's a full barrier and thus what you say is true.