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

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Message-ID: <36c7224c-87a6-aa31-cfaa-06a0b168c68d@huawei.com>
Date:   Mon, 23 May 2022 18:46:14 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     Marco Elver <elver@...gle.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 2022/5/23 16:22, Marco Elver wrote:
> 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.)

I don't know the rules, it seems that some uses 1 spaces, others are 2 
spaces, but most

uses 2 spaces, will update.

> Otherwise it all looks fine to me.
>
> Thanks,
> -- Marco
> .