Re: [PATCH v3] locking/memory-barriers.txt: Improve documentation for writel() example
From: Akira Yokosawa
Date: Thu Oct 06 2022 - 08:31:15 EST
On Thu, 6 Oct 2022 06:44:57 +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
> commit 5846581e3563 ("locking/memory-barriers.txt: Fix broken DMA vs. MMIO ordering example")
> Signed-off-by: Parav Pandit <parav@xxxxxxxxxx>
> - removed redundant description for writeX()
> - updated text for alignment and smaller change lines
> - updated commit log with blank line before signed-off-by line
> - Further improved description of writel() example
> - changed commit subject from 'usage' to 'example'
> - 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 | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index 832b5d36e279..8952fd86c6e6 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -1927,10 +1927,11 @@ 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
> + 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.
> + this guarantee and must not be used here. Hence, writeX() is always
> + preferred.
So I assumed that this last sentence would be removed altogether.
Can you explain the intention of adding it?
IMHO, "preferred" doesn't mean anything in this document.
> 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