linux-kernel - Re: [PATCH v4] locking/memory-barriers.txt: Improve documentation for writel() example

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Message-Id: <12f51033-1461-43f9-8d8d-cd726fbb4758@app.fastmail.com>
Date:   Tue, 18 Oct 2022 08:44:09 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Akira Yokosawa" <akiyks@...il.com>,
        "Parav Pandit" <parav@...dia.com>
Cc:     bagasdotme@...il.com, "Alan Stern" <stern@...land.harvard.edu>,
        parri.andrea@...il.com, "Will Deacon" <will@...nel.org>,
        "Peter Zijlstra" <peterz@...radead.org>, boqun.feng@...il.com,
        "Nicholas Piggin" <npiggin@...il.com>, dhowells@...hat.com,
        j.alglave@....ac.uk, luc.maranget@...ia.fr,
        "Paul E. McKenney" <paulmck@...nel.org>, dlustig@...dia.com,
        "Joel Fernandes" <joel@...lfernandes.org>,
        "Jonathan Corbet" <corbet@....net>, linux-kernel@...r.kernel.org,
        Linux-Arch <linux-arch@...r.kernel.org>,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v4] locking/memory-barriers.txt: Improve documentation for writel()
 example

On Tue, Oct 18, 2022, at 3:37 AM, Akira Yokosawa wrote:
> On 2022/10/18 5:55, Arnd Bergmann wrote:
>> On Mon, Oct 10, 2022, at 12:13 PM, Parav Pandit wrote:
>
> "a barrier" can mean "any barrier", which can include a full barrier
> in theory.
>
> So I'd rather make the substituted text read something like:
>
>   Note that, when using writel(), a prior wmb() or weaker is not
>   needed to guarantee that the cache coherent memory writes have
>   completed before writing to the MMIO region.
>
> In my opinion, "or weaker" is redundant for careful readers who are
> well aware of context of this example, but won't do no harm.

I think that would be more confusing than either of the other variants.

Anything weaker than a full "wmb()" probably makes the driver calling
the writel() non-portable, so that is both vague and incorrect.

The current version works because it specifically mentions the correct
barrier to use, while Parav's version works because it doesn't
make any attempt to name the specific barrier and just states that
adding one is a bad idea regardless.

      Arnd