linux-kernel - RE: [PATCH v1] locking/memory-barriers.txt: Improve documentation for writel() usage

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Message-ID: <PH0PR12MB5481FE2A85C3492AF645E2FADC5B9@PH0PR12MB5481.namprd12.prod.outlook.com>
Date:   Mon, 3 Oct 2022 05:54:53 +0000
From:   Parav Pandit <parav@...dia.com>
To:     Joel Fernandes <joel@...lfernandes.org>
CC:     "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "stern@...land.harvard.edu" <stern@...land.harvard.edu>,
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        "will@...nel.org" <will@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
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Subject: RE: [PATCH v1] locking/memory-barriers.txt: Improve documentation for
 writel() usage

Hi Joel,

> From: Joel Fernandes <joel@...lfernandes.org>
> Sent: Friday, September 30, 2022 7:41 AM
> 
> Hi,
> 
> > On Sep 29, 2022, at 10:04 PM, Parav Pandit <parav@...dia.com> wrote:
> >
> > The cited commit describes that when using writel(), explcit wmb() is
> > not needed. wmb() is an expensive barrier. writel() uses the needed
> > I/O barrier instead of expensive wmb().
> 
> Because you mentioned it in the commit message, Why not mention in the
> documentation text as well that writel() has the needed I/O barrier in it?
>
It is already documented in [1].
Though [1] is doesn’t describe about using I/O barrier.

To keep [1] and above example in sync, and to address Akira's comment [2], it makes sense to drop I/O barrier from the above example and reword the line as below

"Note that, when using writel(), a prior barrier is not needed".
 
[1] https://elixir.bootlin.com/linux/latest/source/Documentation/memory-barriers.txt#L2559
[2] https://lore.kernel.org/lkml/5db465f3-698f-ebee-a668-1740a705ce9c@gmail.com/
> >
> > Hence update the example to be more accurate that matches the current
> > implementation.
> 
> That would make it more accurate, since accuracy is your goal.
> 
Yes. Above modified text looks more aligned to writeX() doc.
Will send v2.