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Date:   Thu, 15 Sep 2022 14:37:57 +0200
From:   "Arnd Bergmann" <arnd@arndb.de>
To:     "Parav Pandit" <parav@nvidia.com>, stern@rowland.harvard.edu,
        parri.andrea@gmail.com, "Will Deacon" <will@kernel.org>,
        "Peter Zijlstra" <peterz@infradead.org>, boqun.feng@gmail.com,
        "Nicholas Piggin" <npiggin@gmail.com>, dhowells@redhat.com,
        j.alglave@ucl.ac.uk, luc.maranget@inria.fr,
        "Paul E. McKenney" <paulmck@kernel.org>, akiyks@gmail.com,
        dlustig@nvidia.com, joel@joelfernandes.org,
        "Jonathan Corbet" <corbet@lwn.net>, linux-kernel@vger.kernel.org,
        Linux-Arch <linux-arch@vger.kernel.org>,
        linux-doc@vger.kernel.org
Subject: Re: [PATCH] locking/memory-barriers.txt: Improve documentation for writel()
 usage
Content-Type: text/plain
Precedence: bulk

On Thu, Sep 15, 2022, at 7:01 AM, Parav Pandit wrote:
> The cited commit [1] describes that when using writel(), explcit wmb()
> is not needed. However, it should have said that dma_wmb() is not
> needed.

Are you sure? As I understand it, the dma_wmb() only serializes
a set of memory accesses, but does not serialized against an MMIO
access, which depending on the CPU architecture may require a
different type of barrier.

E.g. on arm, writel() uses __iowmb(), which like wmb() is defined
as "dsb(x); arm_heavy_mb();", while dma_wmb() is a "dmb(oshst)".

     Arnd