In many cases, especially in networking, it can be beneficial to

know at compile time whether the architecture can do unaligned

accesses efficiently. This patch introduces a new Kconfig symbol

	HAVE_EFFICIENT_UNALIGNED_ACCESS

for that purpose and adds it to the powerpc and x86 architectures.

Also add some documentation about alignment and networking, and

especially one intended use of this symbol.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>

Acked-by: Sam Ravnborg <sam@ravnborg.org>

Acked-by: Ingo Molnar <mingo@elte.hu> [x86 architecture part]

---

v5: rename to HAVE_EFFICIENT_UNALIGNED_ACCESS

v6: small text fixes
I have opted to not introduce any symbol associated with the cost

of unaligned accesses, David Woodhouse once suggested that such a

cost should be combined with a probability of (un-)alignment even

in the get_unaligned/put_unaligned macros and I think this should

be combined with a patch introducing a global cost constant.
Also, this would require architecture changes because if some code

knew the likelihood of unaligned accesses was small enough over

the cost of them, architectures that complain in the trap handle

would still complain in that unlikely case although the code

explicitly made a trade-off based on the cost/probability.
I can roll more architectures that want this into this patch, or you can

merge extra patches once this is in, I don't care either way but the

latter might be quicker/easier.
 Documentation/unaligned-memory-access.txt |   32 +++++++++++++++++++++++++++---

 arch/Kconfig                              |   19 +++++++++++++++++

 arch/powerpc/Kconfig                      |    1

 arch/x86/Kconfig                          |    1

 4 files changed, 50 insertions(+), 3 deletions(-)
--- everything.orig/Documentation/unaligned-memory-access.txt	2008-05-07 21:50:09.000000000 +0200

+++ everything/Documentation/unaligned-memory-access.txt	2008-05-07 21:53:01.000000000 +0200

@@ -218,9 +218,35 @@ If use of such macros i...
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