linux-kernel - Re: [PATCH v4] locking/memory-barriers.txt: Improve documentation for writel() example

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Date:   Tue, 18 Oct 2022 11:05:55 +0100
From:   Will Deacon <will@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Parav Pandit <parav@...dia.com>, bagasdotme@...il.com,
        Alan Stern <stern@...land.harvard.edu>, parri.andrea@...il.com,
        Peter Zijlstra <peterz@...radead.org>, boqun.feng@...il.com,
        Nicholas Piggin <npiggin@...il.com>, dhowells@...hat.com,
        j.alglave@....ac.uk, luc.maranget@...ia.fr,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Akira Yokosawa <akiyks@...il.com>, dlustig@...dia.com,
        Joel Fernandes <joel@...lfernandes.org>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        Linux-Arch <linux-arch@...r.kernel.org>,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v4] locking/memory-barriers.txt: Improve documentation
 for writel() example

On Mon, Oct 17, 2022 at 10:55:00PM +0200, Arnd Bergmann wrote:
> On Mon, Oct 10, 2022, at 12:13 PM, 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")
> >
> > Signed-off-by: Parav Pandit <parav@...dia.com>
> 
> I have no objections, though I still don't see a real need to change
> the wording here.

FWIW, I also don't think this change is necessary. If anything, I'd say
we'd be better off _removing_ the text about writel from this section and
extending the reference to the "KERNEL I/O BARRIER EFFECTS" section,
as you could make similar comments about e.g. readb() and subsequent
barriers.

For example, something like the diff below.

Will

--->8

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 06f80e3785c5..93d9a90b7cfa 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
@@ -1935,18 +1936,15 @@ There are some more advanced barrier functions:
                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.
+     can see it now has ownership.  dma_mb() implies both a dma_rmb() and
+     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();