linux-kernel - Re: [PATCH v2] locking/memory-barriers.txt: Improve documentation for writel() example

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Date:   Thu, 6 Oct 2022 11:52:29 +0900
From:   Akira Yokosawa <akiyks@...il.com>
To:     Parav Pandit <parav@...dia.com>
Cc:     bagasdotme@...il.com, arnd@...db.de, stern@...land.harvard.edu,
        parri.andrea@...il.com, will@...nel.org, peterz@...radead.org,
        boqun.feng@...il.com, npiggin@...il.com, dhowells@...hat.com,
        j.alglave@....ac.uk, luc.maranget@...ia.fr, paulmck@...nel.org,
        dlustig@...dia.com, joel@...lfernandes.org, corbet@....net,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-doc@...r.kernel.org, Akira Yokosawa <akiyks@...il.com>
Subject: Re: [PATCH v2] locking/memory-barriers.txt: Improve documentation for
 writel() example

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@...dia.com>
>>
>> ---
>> 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