[PATCH 17/18] documentation: memory-barriers: clarify relaxed io accessor semantics

[Will Deacon]

This patch extends the paragraph describing the relaxed read io accessors
so that the relaxed accessors are defined to be:

 - Ordered with respect to each other if accessing the same peripheral

 - Unordered with respect to normal memory accesses

 - Unordered with respect to LOCK/UNLOCK operations

Whilst many architectures will provide stricter semantics, ARM, Alpha and
PPC can achieve significant performance gains by taking advantage of some
or all of the above relaxations.

Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
---
 Documentation/memory-barriers.txt | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 556f951f8626..f31c88691ee9 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -2462,10 +2462,15 @@ functions:
      Please refer to the PCI specification for more information on interactions
      between PCI transactions.
 
- (*) readX_relaxed()
-
-     These are similar to readX(), but are not guaranteed to be ordered in any
-     way. Be aware that there is no I/O read barrier available.
+ (*) readX_relaxed(), writeX_relaxed()
+
+     These are similar to readX() and writeX(), but provide weaker memory
+     ordering guarantees. Specifically, they do not guarantee ordering with
+     respect to normal memory accesses (e.g. DMA buffers) nor do they guarantee
+     ordering with respect to LOCK or UNLOCK operations. If the latter is
+     required, an mmiowb() barrier can be used. Note that relaxed accesses to
+     the same peripheral are guaranteed to be ordered with respect to each
+     other.
 
  (*) ioreadX(), iowriteX()
 
-- 
1.9.1

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