Messages in this thread Patch in this message | | | From | Will Deacon <> | Subject | [PATCH 17/18] documentation: memory-barriers: clarify relaxed io accessor semantics | Date | Thu, 17 Apr 2014 14:44:20 +0100 |
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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> --- 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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