linux-kernel - Re: [PATCH v3 1/2] asm-generic: Add memory barrier dma

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Date:   Mon, 23 May 2022 10:22:38 +0200
From:   Marco Elver <elver@...gle.com>
To:     Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, Jonathan Corbet <corbet@....net>,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v3 1/2] asm-generic: Add memory barrier dma_mb()

On Mon, 23 May 2022 at 03:50, Kefeng Wang <wangkefeng.wang@...wei.com> wrote:
>
> The memory barrier dma_mb() is introduced by commit a76a37777f2c
> ("iommu/arm-smmu-v3: Ensure queue is read after updating prod pointer"),
> which is used to ensure that prior (both reads and writes) accesses
> to memory by a CPU are ordered w.r.t. a subsequent MMIO write, this
> is only defined on arm64, but it is a generic memory barrier, let's
> add dma_mb() into documentation and include/asm-generic/barrier.h.
>
> Signed-off-by: Kefeng Wang <wangkefeng.wang@...wei.com>
> ---
>  Documentation/memory-barriers.txt | 11 ++++++-----
>  include/asm-generic/barrier.h     |  8 ++++++++
>  2 files changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index b12df9137e1c..07a8b8e1b12a 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -1894,6 +1894,7 @@ There are some more advanced barrier functions:
>
>   (*) dma_wmb();
>   (*) dma_rmb();
> + (*) dma_mb();
>
>       These are for use with consistent memory to guarantee the ordering
>       of writes or reads of shared memory accessible to both the CPU and a
> @@ -1925,11 +1926,11 @@ There are some more advanced barrier functions:
>       The dma_rmb() allows us guarantee the device has released ownership
>       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.  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.
> +     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.

It seems you've changed that spacing. This document uses 2 spaces
after a sentence-ending '.'. (My original suggestion included the 2
spaces after dots.)

Otherwise it all looks fine to me.

Thanks,
-- Marco