linux-kernel - Re: [PATCH v5] locking/memory-barriers.txt: Improve documentation for writel() example

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Date:   Fri, 28 Oct 2022 10:56:05 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
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, akiyks@...il.com,
        dlustig@...dia.com, joel@...lfernandes.org, corbet@....net,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v5] locking/memory-barriers.txt: Improve documentation
 for writel() example

On Thu, Oct 27, 2022 at 11:10:00PM +0300, Parav Pandit wrote:
> The cited commit describes that when using writel(), explicit wmb()
> is not needed. wmb() is an expensive barrier. writel() uses the needed
> platform specific barrier instead of wmb().
> 
> writeX() section of "KERNEL I/O BARRIER EFFECTS" already describes
> ordering of I/O accessors with MMIO writes.
> 
> Hence add the comment for pseudo code of writel() and remove confusing
> text around writel() and wmb().
> 
> commit 5846581e3563 ("locking/memory-barriers.txt: Fix broken DMA vs. MMIO ordering example")
> 
> Signed-off-by: Parav Pandit <parav@...dia.com>

Hearing no immediate objections, I have pulled this in for further
review.  If all goes well, I intend to submit this during the upcoming
v6.2 merge window.

							Thanx, Paul

> ---
> changelog:
> v4->v5:
> - Used suggested documentation update from Will
> - Added comment to the writel() pseudo code example
> - updated commit log for newer changes
> v3->v4:
> - further trimmed the documentation for redundant description
> v2->v3:
> - removed redundant description for writeX()
> - updated text for alignment and smaller change lines
> - updated commit log with blank line before signed-off-by line
> 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 | 22 +++++++++++-----------
>  1 file changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index 06f80e3785c5..e698093bade1 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -1910,7 +1910,8 @@ There are some more advanced barrier functions:
>  
>       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. See Documentation/core-api/dma-api.rst file for more
> +     information about consistent memory.
>  
>       For example, consider a device driver that shares memory with a device
>       and uses a descriptor status value to indicate if the descriptor belongs
> @@ -1931,22 +1932,21 @@ There are some more advanced barrier functions:
>  		/* assign ownership */
>  		desc->status = DEVICE_OWN;
>  
> -		/* notify device of new descriptors */
> +		/* Make descriptor status visible to the device followed by
> +		 * notify device of new descriptor
> +		 */
>  		writel(DESC_NOTIFY, doorbell);
>  	}
>  
> -     The dma_rmb() allows us guarantee the device has released ownership
> +     The dma_rmb() allows us to guarantee that 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.  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
> -     more information on consistent memory.
> +     a dma_wmb().
> +
> +     Note that the dma_*() barriers do not provide any ordering guarantees for
> +     accesses to MMIO regions.  See the later "KERNEL I/O BARRIER EFFECTS"
> +     subsection for more information about I/O accessors and MMIO ordering.
>  
>   (*) pmem_wmb();
>  
> -- 
> 2.26.2
>