linux-kernel - Re: [PATCH v3] locking/memory-barriers.txt: Improve documentation for writel() example

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Message-ID: <2dedeb9a-279d-8756-cb74-6d0919f0a02e@gmail.com>
Date:   Thu, 6 Oct 2022 21:30:32 +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
Subject: Re: [PATCH v3] locking/memory-barriers.txt: Improve documentation for
 writel() example

Hi,

On Thu, 6 Oct 2022 06:44:57 +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
> platform specific 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:
> v2->v3:
> - removed redundant description for writeX()
> - updated text for alignment and smaller change lines
> - updated commit log with blank line before signed-off-by line
> v1->v2:
> - Further improved description of writel() example
> - changed commit subject from 'usage' to 'example'
> v0->v1:
> - Corrected to mention I/O barrier instead of dma_wmb().
> - removed numbered references in commit log
> - corrected typo 'explcit' to 'explicit' in commit log
> ---
>  Documentation/memory-barriers.txt | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index 832b5d36e279..8952fd86c6e6 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -1927,10 +1927,11 @@ There are some more advanced barrier functions:
>       before we read the data from the descriptor, and the dma_wmb() allows
>       us to guarantee the data is written to the descriptor before the device
>       can see it now has ownership.  The dma_mb() implies both a dma_rmb() and
> -     a dma_wmb().  Note that, when using writel(), a prior wmb() is not needed
> +     a dma_wmb().  Note that, when using writel(), a prior barrier is not needed
>       to guarantee that the cache coherent memory writes have completed before
>       writing to the MMIO region.  The cheaper writel_relaxed() does not provide
> -     this guarantee and must not be used here.
> +     this guarantee and must not be used here. Hence, writeX() is always
> +     preferred.
So I assumed that this last sentence would be removed altogether.
Can you explain the intention of adding it?

IMHO, "preferred" doesn't mean anything in this document.

        Thanks, Akira

>  
>       See the subsection "Kernel I/O barrier effects" for more information on
>       relaxed I/O accessors and the Documentation/core-api/dma-api.rst file for