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

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Date:   Tue, 18 Oct 2022 09:49:34 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Akira Yokosawa" <akiyks@...il.com>,
        "Parav Pandit" <parav@...dia.com>
Cc:     "Bagas Sanjaya" <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 9:40 AM, Akira Yokosawa wrote:
> On Tue, 18 Oct 2022 08:44:09 +0200, Arnd Bergmann wrote:
>> 
>> Anything weaker than a full "wmb()" probably makes the driver calling
>> the writel() non-portable, so that is both vague and incorrect.
>
> Do you mean there is a writel() implementation somewhere in the kernel
> which doesn't guarantee an implicit wmb() before MMIO write?

There are lots of those, but that's not what I meant. E.g. on x86,
writel() does not imply a full wmb() but still guarantees serialization
between DMA and the register access.

> Or do you mean my version is confusing because it can imply a weaker
> write barrier is sufficient before writel_relaxed()?

That's what I meant, yes. On a lot of architectures, it is sufficient
to have something weaker than wmb() before writel_relaxed(), especially
on anything that defines writel_relaxed() to be the same as writel(),
any barrier would technically work. On arm32, using __iowmb() would be
sufficient, and this can be less than a full wmb() but again it's
obviously not portable. These details should not be needed in the
documentation.

      Arnd