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

Paul E. McKenney
2019-01-09T21:08:03Z

From: Will Deacon <will.deacon@arm.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@kernel.crashing.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: David Laight <David.Laight@ACULAB.COM>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: <linux-arch@vger.kernel.org>
Cc: <linux-doc@vger.kernel.org>
Cc: <linux-kernel@vger.kernel.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.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