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

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Date:   Thu, 29 Sep 2022 22:10:31 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
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,
        akiyks@...il.com, dlustig@...dia.com, corbet@....net,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v1] locking/memory-barriers.txt: Improve documentation for writel() usage

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?

> 
> Hence update the example to be more accurate that matches the current
> implementation.

That would make it more accurate, since accuracy is your goal.

thanks,

 - Joel


> 
> 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().
> - 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(-)
> 
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index 832b5d36e279..2d77a7411e52 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
> -     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.
> +     a dma_wmb().  Note that, when using writel(), a prior I/O 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. Hence, writeX() is always
> +     preferred.
> 
>      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
> -- 
> 2.26.2
>