documentation: memory-barriers: clarify relaxed io accessor semantics
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@infradead.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 22a969c..4af4cea 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -2465,10 +2465,15 @@
Please refer to the PCI specification for more information on interactions
between PCI transactions.
- (*) readX_relaxed()
+ (*) readX_relaxed(), writeX_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.
+ 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()