linux-kernel - Re: [PATCH] docs/memory-barriers.txt: Enforce heavy ordering for port I/O accesses

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Date:   Mon, 26 Nov 2018 11:42:05 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Will Deacon <will.deacon@....com>
Cc:     corbet@....net, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Laight <David.Laight@...LAB.COM>,
        Alan Stern <stern@...land.harvard.edu>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] docs/memory-barriers.txt: Enforce heavy ordering for
 port I/O accesses

On Mon, Nov 26, 2018 at 04:52:14PM +0000, Will Deacon wrote:
> David Laight explains:
> 
>   | A long time ago there was a document from Intel that said that
>   | inb/outb weren't necessarily synchronised wrt memory accesses.
>   | (Might be P-pro era). However no processors actually behaved that
>   | way and more recent docs say that inb/outb are fully ordered.
> 
> This also reflects the situation on other architectures, the the port
> accessor macros tend to be implemented in terms of readX/writeX.
> 
> Update Documentation/memory-barriers.txt to reflect reality.
> 
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: David Laight <David.Laight@...LAB.COM>
> Cc: Alan Stern <stern@...land.harvard.edu>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Signed-off-by: Will Deacon <will.deacon@....com>

Queued, with the addition of Cc: of LKML, linux-arch, and linux-docs,
thank you!

(Otherwise, these lists can get lost when I send out the LKMM series.)

							Thanx, Paul

> ---
> 
> Just remembered I had this patch kicking around in my tree...
> 
>  Documentation/memory-barriers.txt | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index c1d913944ad8..0c34c5dac138 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -2619,10 +2619,8 @@ functions:
>       intermediary bridges (such as the PCI host bridge) may not fully honour
>       that.
> 
> -     They are guaranteed to be fully ordered with respect to each other.
> -
> -     They are not guaranteed to be fully ordered with respect to other types of
> -     memory and I/O operation.
> +     They are guaranteed to be fully ordered with respect to each other and
> +     also with respect to other types of memory and I/O operation.
> 
>   (*) readX(), writeX():
> 
> -- 
> 2.1.4
>