linux-kernel - [PATCH RFC LKMM 5/7] docs/memory-barriers.txt: Enforce heavy ordering for port I/O accesses

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Date:   Wed,  9 Jan 2019 13:07:46 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        mingo@...nel.org
Cc:     stern@...land.harvard.edu, parri.andrea@...il.com,
        will.deacon@....com, peterz@...radead.org, boqun.feng@...il.com,
        npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, willy@...radead.org,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Laight <David.Laight@...LAB.COM>,
        linux-doc@...r.kernel.org,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>
Subject: [PATCH RFC LKMM 5/7] docs/memory-barriers.txt: Enforce heavy ordering for port I/O accesses

From: Will Deacon <will.deacon@....com>

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: <linux-arch@...r.kernel.org>
Cc: <linux-doc@...r.kernel.org>
Cc: <linux-kernel@...r.kernel.org>
Signed-off-by: Will Deacon <will.deacon@....com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
---
 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 1c22b21ae922..a70104e2a087 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():
 
-- 
2.17.1