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

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Date:   Wed, 28 Mar 2018 10:57:11 -0700
From:   Tony Luck <tony.luck@...el.com>
To:     Sinan Kaya <okaya@...eaurora.org>
Cc:     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>,
        Peter Zijlstra <peterz@...radead.org>,
        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 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.

-Tony