linux-kernel - [PATCH v1] locking/memory-barriers.txt: Improve documentation for writel() usage

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Date:   Fri, 30 Sep 2022 05:03:55 +0300
From:   Parav Pandit <parav@...dia.com>
To:     <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>, <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>
CC:     Parav Pandit <parav@...dia.com>
Subject: [PATCH v1] locking/memory-barriers.txt: Improve documentation for writel() usage

The cited commit describes that when using writel(), explcit wmb()
is not needed. wmb() is an expensive barrier. writel() uses the needed
I/O 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")
Signed-off-by: Parav Pandit <parav@...dia.com>

---
changelog:
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 | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 832b5d36e279..2d77a7411e52 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
-     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 I/O 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.
 
      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
-- 
2.26.2