On Thu, 6 Oct 2022 11:48:04 +0900, Akira Yokosawa wrote:
> Hi,
>
> Maybe I'm too nit-picky, but see below:
>
> On Wed, 5 Oct 2022 13:47:49 +0300, Parav Pandit wrote:
>> The cited commit describes that when using writel(), explcit wmb()
>> is not needed. wmb() is an expensive barrier. writel() uses the needed
>> platform specific barrier instead of expensive wmb().
>>
>> Hence update the example to be more accurate that matches the current
>> implementation.
>>
>> commit 5846581e3563 ("locking/memory-barriers.txt: Fix broken DMA vs. MMIO ordering example")
>
> You can cite the commit in the Changelog text.
> Just say:
>
> Commit 5846581e3563 ("locking/memory-barriers.txt: Fix broken DMA vs.
> MMIO ordering example") describes that when using writel(), ...
>
> Also, a blank line is needed above S-o-b tags as a delimiter.
>
>> Signed-off-by: Parav Pandit <parav@xxxxxxxxxx>
>>
>> ---
>> changelog:
>> v1->v2:
>> - Further improved description of writel() example
>> - changed commit subject from 'usage' to 'example'
>> 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 | 10 ++++++----
>> 1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
>> index 832b5d36e279..49e1433db407 100644
>> --- a/Documentation/memory-barriers.txt
>> +++ b/Documentation/memory-barriers.txt
>> @@ -1927,10 +1927,12 @@ 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 barrier is not
> If you permit a slightly long line here, this hunk would be much easier to
> compare:
>
> + a dma_wmb(). Note that, when using writel(), a prior 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 which inserts needed platform specific barrier before writing to
> + the specified MMIO region.
>
> That said, I don't feel comfortable with the sentence you added.
> It looks to me it is redundant because such a guarantee of writeX() is already
> covered in the section of "KERNEL I/O BARRIER EFFECTS".
> See the relevant explanation quoted below:
>
> 3. A writeX() by a CPU thread to the peripheral will first wait for the
> completion of all prior writes to memory either issued by, or
> propagated to, the same thread. This ensures that writes by the CPU
> to an outbound DMA buffer allocated by dma_alloc_coherent() will be
> visible to a DMA engine when the CPU writes to its MMIO control
> register to trigger the transfer.
>
> Also please not that this document should not describe any implementation
I meant note
> details of those accessors. This document is not meant as an implementation
> guide, but a guide for kernel developers who use them. This is clearly
> mentioned in "DISCLAIMER" at the top of this file.
>
> Thanks, Akira
>
>> + 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 which inserts needed platform specific barrier before writing to
>> + the specified MMIO region.
>>
>> 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
