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    Subject[PATCH v3 16/17] documentation: memory-barriers: clarify relaxed io accessor semantics
    Date
    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 a4de88fb55f0..6b2b4d735a5b 100644
    --- a/Documentation/memory-barriers.txt
    +++ b/Documentation/memory-barriers.txt
    @@ -2461,10 +2461,15 @@ 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()

    --
    2.1.0


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