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From:   Joel Fernandes <joel@joelfernandes.org>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH v1] locking/memory-barriers.txt: Improve documentation for writel() usage
Date:   Thu, 29 Sep 2022 22:10:31 -0400
Message-Id: <CCD8EFA1-D0CF-4E57-905C-E7CA8E4DA56F@joelfernandes.org>
References: <20220930020355.98534-1-parav@nvidia.com>
Cc:     bagasdotme@gmail.com, arnd@arndb.de, stern@rowland.harvard.edu,
        parri.andrea@gmail.com, will@kernel.org, peterz@infradead.org,
        boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com,
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        linux-doc@vger.kernel.org
In-Reply-To: <20220930020355.98534-1-parav@nvidia.com>
To:     Parav Pandit <parav@nvidia.com>
Precedence: bulk

Hi,

> On Sep 29, 2022, at 10:04 PM, Parav Pandit <parav@nvidia.com> wrote:
>=20
> =EF=BB=BFThe 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 docum=
entation text as well that writel() has the needed I/O barrier in it?

>=20
> 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


>=20
> commit 5846581e3563 ("locking/memory-barriers.txt: Fix broken DMA vs. MMIO=
 ordering example")
> Signed-off-by: Parav Pandit <parav@nvidia.com>
>=20
> ---
> 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(-)
>=20
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barr=
iers.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 devi=
ce
>      can see it now has ownership.  The dma_mb() implies both a dma_rmb() a=
nd
> -     a dma_wmb().  Note that, when using writel(), a prior wmb() is not n=
eeded
> -     to guarantee that the cache coherent memory writes have completed be=
fore
> -     writing to the MMIO region.  The cheaper writel_relaxed() does not p=
rovide
> -     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 compl=
eted
> +     before writing to the MMIO region.  The cheaper writel_relaxed() doe=
s not
> +     provide this guarantee and must not be used here. Hence, writeX() is=
 always
> +     preferred.
>=20
>      See the subsection "Kernel I/O barrier effects" for more information o=
n
>      relaxed I/O accessors and the Documentation/core-api/dma-api.rst file=
 for
> --=20
> 2.26.2
>=20