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

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Date:   Fri, 30 Sep 2022 12:11:29 +0900
From:   Akira Yokosawa <akiyks@...il.com>
To:     Parav Pandit <parav@...dia.com>
Cc:     bagasdotme@...il.com, arnd@...db.de, stern@...land.harvard.edu,
        parri.andrea@...il.com, will@...nel.org, peterz@...radead.org,
        boqun.feng@...il.com, npiggin@...il.com, dhowells@...hat.com,
        j.alglave@....ac.uk, luc.maranget@...ia.fr, paulmck@...nel.org,
        dlustig@...dia.com, joel@...lfernandes.org, corbet@....net,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-doc@...r.kernel.org, Akira Yokosawa <akiyks@...il.com>
Subject: Re: [PATCH v1] locking/memory-barriers.txt: Improve documentation for
 writel() usage

Hi, 

On Fri, 30 Sep 2022 05:03:55 +0300, Parav Pandit 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().
> 
> Hence update the example to be more accurate that matches the current
> implementation.
> 
> commit 5846581e3563 ("locking/memory-barriers.txt: Fix broken DMA vs. MMIO ordering example")
> Signed-off-by: Parav Pandit <parav@...dia.com>
> 
> ---
> changelog:
> v0->v1:
> - Corrected to mention I/O barrier instead of dma_wmb().
I don't think dma_wmb() and wmb() belong to "I/O barrier" as far as
memory-barriers.txt is concerned. They are listed in the "CPU MEMORY
BARRIERS" section. dma_wmb() belongs to "advanced barrier functions".

You see, writel() is one of the functions listed in the "KERNEL I/O
BARRIER EFFECTS" section.

Please be consistent with the word choice of the doc you are modifying,
so that any further confusion can be avoided in this infamously 
hard-to-follow document. :-)

Regards,
Akira

> - removed numbered references in commit log
> - corrected typo 'explcit' to 'explicit' in commit log
> ---
>  Documentation/memory-barriers.txt | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
[...]