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

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Date:   Thu, 6 Oct 2022 11:48:04 +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

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
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