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Date:   Tue, 18 Oct 2022 16:40:06 +0900
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Subject: Re: [PATCH v4] locking/memory-barriers.txt: Improve documentation for
 writel() example
To:     Arnd Bergmann <arnd@arndb.de>, Parav Pandit <parav@nvidia.com>
Cc:     bagasdotme@gmail.com, Alan Stern <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>, dlustig@nvidia.com,
        Joel Fernandes <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, Akira Yokosawa <akiyks@gmail.com>
References: <20221010101331.29942-1-parav@nvidia.com>
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From:   Akira Yokosawa <akiyks@gmail.com>
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On Tue, 18 Oct 2022 08:44:09 +0200, Arnd Bergmann wrote:
> 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.

Do you mean there is a writel() implementation somewhere in the kernel
which doesn't guarantee an implicit wmb() before MMIO write?

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

I'm confused...

        Thanks, Akira

> 
> 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