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From:   Tony Luck <tony.luck@intel.com>
Date:   Wed, 28 Mar 2018 10:57:11 -0700
Message-ID: <CA+8MBb+61AhMkbo8xtd3Y0fY8xRUPD+kUsEvWgOz94KGEuovOw@mail.gmail.com>
Subject: Re: [PATCH] docs/memory-barriers.txt: Fix broken DMA vs MMIO ordering example
To:     Sinan Kaya <okaya@codeaurora.org>
Cc:     Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Will Deacon <will.deacon@arm.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        linux-doc@vger.kernel.org,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Arnd Bergmann <arnd@arndb.de>, Jason Gunthorpe <jgg@ziepe.ca>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        "linux-ia64@vger.kernel.org" <linux-ia64@vger.kernel.org>
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On Wed, Mar 28, 2018 at 6:02 AM, Sinan Kaya <okaya@codeaurora.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.

-Tony