linux-kernel - Re: [PATCH v2 1/2] Documentation/barriers: Add memory barrier dma

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Date:   Fri, 20 May 2022 19:14:33 +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>,
        <paulmck@...nel.org>, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 1/2] Documentation/barriers: Add memory barrier
 dma_mb()


On 2022/5/20 18:08, Marco Elver wrote:
> On Fri, May 20, 2022 at 11:15AM +0800, Kefeng Wang 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.
>>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@...wei.com>
>> ---
>>   Documentation/memory-barriers.txt | 5 ++++-
>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
>> index b12df9137e1c..1eabcc0e4eca 100644
>> --- a/Documentation/memory-barriers.txt
>> +++ b/Documentation/memory-barriers.txt
>> @@ -1894,10 +1894,13 @@ 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
>> -     DMA capable device.
>> +     DMA capable device, in the case of ensure the prior (both reads and
>> +     writes) accesses to memory by a CPU are ordered w.r.t. a subsequent
>> +     MMIO write, dma_mb().
>>   
> I think this is out of place; this explanation here is not yet
> elaborating on either. Elaboration on dma_mb() should go where
> dma_rmb() and dma_wmb() are explained.
>
> Something like this:
>
> ------ >8 ------
>
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index b12df9137e1c..fb322b6cce70 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.
>   
>        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
>
> ------ >8 ------
Thanks， will use above explanation.
> Also, now that you're making dma_mb() part of the official API, it might
> need a generic definition in include/asm-generic/barrier.h, because
> as-is it's only available in arm64 builds.

Ok, it's good to add the dma_mb() and __dma_mb  definition with a 
separate patch

into include/asm-generic/barrier.h.

>
> Thoughts?
>
> Thanks,
> -- Marco
> .